Okay, so first of all, thank you for providing us with truly interesting takes on the characters (and I'll also say very funny and surprising ones for a canon purist like me, given your propensity to enjoy and seriously consider the most outlandish Harry Potter ships one could think of). That said, I am a self-appointed Slytherin locket expert (locket that I sometimes refer to as my archenemy because I am dramatic like that), so you will excuse me if I disagree somewhat with your take and offer my reading of what went on with the locket.
To me, it's a grave mistake to consider the mental torture Ron went through with Voldemort-locket only through the lens of jealousy (or even through the lens of jealousy at all, really), and more particularly romantic jealousy. To me, his fear that Harry and Hermione might end up together is only the logical consequence of his low self-esteem, his deep insecurities regarding his self-worth, his place in his family and in the world, his fear of rejection and abandonment and his fear of invisibility. I will even say that it's secondary, that it's really not what was the crux of the matter here.
You wrote that his fears regarding his place in his own family, his relationship with his mother and how Harry fits in all of this should be viewed as "an additional strand" to his fear that Harry and Hermione would end up together. And yet, the very first thing that comes out of Voldemort-locket's mouth is that Ron has always been "least loved" by his mother, who allegedly "craved a daughter".
The idea that Molly "craved a daughter" was new to us readers, though it's not difficult to imagine how Ron came to that conclusion or how someone made him think that. His complex relationship with his mother, however, has been there since the very beginning, before he even met Harry, something that was hinted at from the very first time we're introduced to him. It's central to understand why Ron had self-esteem issues to begin with.
Before I dive into this, I want to say I'm not here to paint Molly as a bad mother or person; she's fiercely caring and kind, she raised seven amazing children, and she called Bellatrix Lestrange a bitch to her face before eliminating her. But even great mothers aren't perfect, because no one is, and people make mistakes.
Ron, as he said himself during his first encounter with Harry, is the sixth son in a family of seven. That means, as he told Harry, that he only gets the hand-me-downs of his brothers, and that he has trouble defining who he is, in the sense that he feels that no matter what he does, it'll never be good enough to be seen by his parents - so it speaks at once about his difficulty finding his own place in his family, and the fact that he lacks attention from his parents, both in terms of quantity and quality - and more particularly, from his mother, the one parent he spends most time with.
In fact, we're shown a quiet example of that not long after, as Ron looks at one of his dry sandwiches while Harry has all his wizarding sweets next to him and says: "She always forgets I don't like corned beef." Right after, Harry suggests that he could swap some of his sweets for one of Ron's sandwich, but Ron, who's not going to mislead his new friend on food quality, tells him that Harry shouldn't take his sandwiches because "it's all dry". And because that could paint his mother in a bad light in his mind, he adds that it's because Molly doesn't have a lot of time with the number of children there is at home that his sandwiches aren't good.
It tells you a lot of things about Ron's relationship with Molly in a few words. He clearly loves his mother and is protective of her, but there is also a quiet sadness that comes from him feeling somewhat invisible to her. He's internalized the idea that she doesn't have much time to give to him, that he's one among others, and all of those are stated as facts. It's interesting to see that to him, Molly "always" forgetting about his food preferences (or his colour preferences, which he'll comment on at Christmas of their first year), or not having a lot of attention to give to him specifically are facts of life that don't show anything necessarily bad about her, but her making bad sandwiches could.
In fact, when you look at it more closely, the "everyone" Ron refers to when he says "everyone expects me to do as well as the others, but if I do, it's no big deal, because they did it first" could be in particular applied to Molly. One of Molly's biggest flaws as a mother is her tendency to compare her children and their accomplishments. Five years after that first encounter with Harry, for example, when Ron is appointed Prefect, she is at first shocked before telling Ron that "everyone in the family" was a Prefect, referring to a sort of familial tradition that Ron somehow had to follow to be part of them and be on their level; it becomes something that's not just Ron's accomplishment, but something that was "ought to be", and somehow it becomes... not that big a deal, despite Molly's excitement. Ron then is just "another Weasley" once again, invisible in a mass.
I've seen very few people comment on the difference between Ron and Harry's first year of life, even if I think it had a definitive incidence on their development. Before people take out their scythes and torches, I'm not trying to insinuate that Harry's life was not terribly impacted by the abuse and neglect he suffered at the hands of the Dursleys, or that Ron's experience is comparable - it's more that Harry's life would've been much worse had he not spent about a year and three months being dotted on and adored by his parents for whom he was the firstborn.
Now Ron's first year of life had to be different. For starters, Molly already had five children to take care of when he was born, among which two-year old Fred and George, who had to be menaces even when they were babies. Then, she got pregnant with Ginny, and navigating a pregnancy while she already had children to take care of (and probably at least two rowdy children) means that she logically had a lot less time to give to baby Ron. Add on top the fact that Molly lost her brothers who were killed by Death Eaters in 1981 (so after Ron was born), plus of course the financial issues of the family (which before Ron's third year at school were dire, since just buying hand-me-down school furniture meant emptying their bank account the year before that), and you have a little baby who didn't spend much time truly receiving the attention he needed.
This is of course not Molly's (or Arthur's) fault, but this wasn't corrected as years wore one, and this is how Ron ended up looking at the Mirror of Erised, and seeing himself as a Head Boy and captain of Quidditch winning a Quidditch cup. Those things are only a representation of what he actually craved: being seen for himself, and even more so by his parents. And doing what his brothers had done but all at the same time was the only thing his childish mind could see would grant him what he wanted. He's internalized quite young the idea that he has to earn attention.
Of course, it didn't go that way at all. Ron became the best friend of Harry and Hermione, and became even more invisible. A lot of his formative experiences as a teenager actually worsened his self-esteem issues. In the classroom Hermione was the one who shone, but I don't think it really was a problem for Ron in itself, unlike the way people ignored him or shoved him aside to focus on Harry - even though, as Hermione pointed out, he "put up with it, and he never mention[ed] it" (or he did but in a joking manner, like this time he said the Ministry would have to dig him up first if they wanted to punish him for blowing up an Aunt because his mother would kill him first). There's only one example I can think of in the books of Ron being slightly bitter on screen about Harry receiving attention he didn't, and that was in The Goblet of Fire, right before the Yule Ball when he told Harry that he wouldn't have any trouble finding someone to go to the Ball with (his reaction to Slughorn favouring Harry and Hermione was a bit different, but I'll comment on that later).
However, being dismissed for Harry's sake clearly stung more when it came from family. Ron first comments on it in The Prisoner of Azkaban, when he's shocked and clearly nettled that Fred and George gave the Marauder's Map to Harry instead of him, "their brother".
Then there is also the difference between the way Molly treats Harry and the way Molly treats Ron. Again, this isn't an attack on Molly's character; we can all understand why Molly reacted the way she did to Harry, a kind child who didn't want to be a bother to the family who welcomed him as one of their own, who was orphaned from a young age and treated terribly by his relatives, and kept going through terrible experiences. She knew he needed tenderness, love, family, and she gave it to him whenever she could.
But there is as a result a sort of discrepancy: Molly treats Harry a bit more carefully, more delicately, almost with kid's gloves compared to Ron. It reminds me a bit of the way Fred commented during Harry's first Christmas at Hogwarts that Molly made "more of an effort" towards people who aren't family when he saw that Harry's sweater was more beautiful than his own or that of his brothers. Obviously, in this case Harry became family to Molly, but the point stands: Ron is the kid that Molly raised, the one she is charged to discipline and guide, the one she's responsible for, and it's obvious to her he knows he's loved; while Harry is the kid she comes to love like a son but that she can only show love to a few days or weeks a year, that she's not responsible for, and that she doesn't have to discipline.
There is this scene in the Goblet of Fire that shows a bit what I'm talking about, in which Ron discovers the dress robes Molly got for him. Ron is horror-struck upon learning they're his, and says he'll never wear them. Molly replies crossly that "everyone wears that", and when Ron retorts he'd rather go starkers, Molly tells him she got dress robes for Harry, too, and asks Harry to show those to him. When Harry takes his out, Molly comments fondly that she chose green ones to bring out the colour of his eyes. Ron replies angrily that Harry's are okay and asks why he couldn't have something similar. Molly is forced to admit she didn't have the money for it, and snaps when Ron still says he'll never wear the dress robes, telling Harry to take a picture of him if he goes naked because she could use a laugh, before leaving the room and slamming the door behind her.
Of course, it is also because Ron himself has trouble seeing his own worth and because his school experiences do not help with that it becomes worse. There is a definitive escalation in regard to the depth of his problems. In the Prisoner of Azkaban, Ron comments casually on the differences between Harry and he, and clearly enjoys attention when people give it to him after he sees Sirius above his bed. By the Goblet of Fire, Ron starts to crave attention and validation as a way to reassure himself about his own self-worth: he desperately tries to have Moody tell him that, just like Harry and Hermione, he could be a good Auror, is as I said a bit bitter that Harry will easily find someone to go to the Ball with, even lies after the Second Task for a brief moment to make himself out to be heroic (escalating the tale when he gets more attention, and giving up the act the moment Hermione comments on his lying).
Yet it's denied to him: the fake Moody doesn't praise him, and of course the attention he gets after the Second Task is based on lies so wanes quickly after Ron goes back to telling the truth. On top of that, he faces a lot of humiliations: he’s much more prone to be ridiculous under the power of the Veelas than Harry, discovers how ridiculous his dress robes are compared to Harry's, he is humiliated by Draco Malfoy in the Hogwarts Express, is soaked and drenched in water when he arrives at Hogwarts because Peeves drops “a large, red, water-filled balloon” onto his head, is once again humiliated in the Great Hall by Draco Malfoy who insults his parents, makes a fool of himself in front of Fleur Delacour to his greatest shame, has to go to the Yule Ball in his hideous dress robes and discovers his feelings for Hermione the very moment she's courted and then dates the wizarding Ronaldo (whom Ron was an idol of and spent the little money he had on!).
There's also this moment of humiliation that comes right after finding out that the money he gave to Harry to reimburse him at the Quidditch World Cup disappeared because it was Leprechaun gold. At the Quidditch World Cup, he was embarrassed but still happy when Harry offered him the Omniculars, upon the condition that Harry wouldn't give him any Christmas gift. And he was really glad to have the opportunity to give it back to him. So of course it touched him in his dignity when he found out that this money was fake; it's at this moment that Ron comments: “Must be nice…to have so much money you don’t notice if a pocketful of Galleons goes missing", and says "I hate being poor" - it's less about being jealous of Harry's money, and way more about feeling your dignity being taken away from you because of your poverty.
So when Order of the Phoenix arrives, it is already cemented in Ron's brain that he's unworthy, and clearly less worthy than Harry and Hermione. When he's appointed as a Prefect, he doesn't quite believe it's true, the Twins make fun of him and say out loud that to them it was obvious Harry would get it (especially Fred who says that "no one in their right mind would make Ron a Prefect"...sorry Fred I still love you), he doesn't defend himself from them, and though he's a bit defiant when Hermione is also shocked and has a lot of difficulty to recover from it but doesn't treat it as a mistake on her part, he never forgets her surprise (the "Always a tone of surprise" comes from that moment !). He admits that he never thought he could get the Prefect badge when Harry finally congratulates him, but even after he makes a lot of effort to ensure Harry isn't left out, conscious that it hurt Harry even if Harry didn't tell him so; there's this moment right before getting on the Hogwarts Express when Ron at first doesn't want to look at Harry when Hermione tells him they have to go to a Prefect meeting, and goes on to say that he'd rather be with Harry, that it's a pain to go there.
Then he feels presumptuous even trying to get on the Quidditch team - and this word is important, because it's the same one the locket will say to him three years later, albeit in a more general sense. He doesn't tell his best friends that he's training in secret, and when he's found out by Harry he expects him to laugh at him and even blinks when Harry doesn't. You should look at this moment and see a parallel of what the locket told Ron, when he took on the form of his two best friends and had them saying they were mocking him behind his back - it's the same idea.
During his first training, he apologizes every time he drops the ball. And his anxiety days before the first match spikes to a level that's worrying: he shakes from head to toe when he's mocked, drops his things, goes to the toilets to throw up. Of course, the first two matches go terribly, because of the public bullying and humiliation he's forced to endure. This moment isolates Ron: after the first match, he spends the day alone in the snow, depressed, and no one comes looking for him (except Hermione, but she didn't find him); after the second match, he's left on his own during the Gryffindor Party, while everyone looks from afar, sorry for him but some are also sure it'd be best for him to quit. And Ron believes that, too, and thinks it'd be better for the team if he quit (only Angelina doesn't let him...I love Angelina).
You don't see the parallel with "We were better without you, happier without you" ? You don't see a pattern forming ? The entire Quidditch arc in the fifth book is a microcosm of what went on with the locket, with of course much less big stakes, and way less dramatic circumstances (no war, no family in danger, no hunger, no cold, no near-death injury to recover from without medication, and no mental torture).
The Half-Blood Prince is only a continuation of the now established fact that Ron thinks himself less worthy, less lovable, ridiculous. Slughorn embodies on his own Ron's insecurity of being invisible, since he literally treats Ron as if he wasn't there while fawning upon Harry and Hermione: it's what truly stings for Ron; it's not just Harry and Hermione receiving attention he doesn't, it's him being deemed unworthy of it, and excluded as a result.
For about three to four weeks in this book Ron masks his internalized feelings of unworthiness/self-loathing/hurt feelings due to thinking others deem him as inferior with anger, engaging in a self-destructive pattern that will find its way back when he'll be tortured by the locket in the tent and starts a fight with Harry. The best example of that for me is when he "bellow[s] at everybody" so much during a training session that Demelza Robins cries, and yet sags the moment everyone leaves before saying "I resign. I'm pathetic." There's also him shouting at a transfigured Goyle/Crabbe after his fight with Ginny during which he felt humiliated, him getting snarky about the Slug Club in herbology, also him imitating Hermione after she said that him saving all his goals was evidence Harry had put Felix Felicis in his drinks.
I'm not going to go into why Ron dated Lavender because I wrote a lengthy essay about it, but all the paragraphs I wrote so far go to show this: Ron's mental torture with Voldemort-locket was entirely focused on his feelings of unworthiness, his fear of invisibility and rejection, and I'd almost say that his fear that Harry and Hermione would end up together is an afterthought in all this, definitely small potatoes compared to the rest.
The locket took the forms of Harry and Hermione for several reasons. One, they're the people Ron loves most in the world. Two, they're the ones he'd left (though it's not his fault) and the ones he was coming back to. Three, Harry is the one he's been compared to the most during all his teenage years, the one he felt he always fell short to, an embodiment of his feeling of inferiority; Hermione is the girl he loves, and she represents his hope for love in general as well.
When the locket tells Ron that Harry and Hermione were mocking "his stupidity, his cowardice, his presumption", that's because that's the way Ron saw himself, and because there's a part of Ron that expected them to - not because they'd be together as a couple, but because Harry and Hermione as individuals had to despise him. When the locket has Riddle-Hermione saying right after "Who could look at you, who would ever look at you, beside Harry Potter? What have you ever done, compared with the Boy Who Lived ?", that's because he thought he was worthy of no one's love and that he was invisible in general - but it hurt most if it came from Hermione.
When the locket had Riddle-Harry and Riddle-Hermione say they were "better without [him]", "happier without [him]", "glad of [his] absence", it was because there was a part of Ron that genuinely thought this was true in general, and not because he imagined Harry and Hermione would spend all their alone time making out.
And when the locket started by commenting on Ron's relationship with Molly, and carried on with the way Harry could maybe replace him, that's because his relationship his Molly is the basis upon which his self-esteem issues developed, the very origin of Ron's insecurities: Ron thought he was less worthy of his mother's love, and based on his inferiority complex towards Harry, his feeling of being invisible inside his own family, and the way Molly treated Harry with fondness, there was but a small step before figuring that maybe Harry would be someone whom Molly would finally feel proud of to have as a son (I do not want imagine how much Molly would've sobbed if she'd been there...).
But Ron destroying the locket isn't Ron banishing "his jealousy" - it's not about that. Why would he fall in the snow and sob if it was a moment of triumph ? No, if you remember, Ron got temporarily possessed right before destroying the locket, and Harry thought Ron would kill him. That's what it was all about: Ron wasn't banishing his self-esteem issues, he was saying that no matter how much they affected him, they'd never be bigger than his love for his family and friends, of his desire to do what was right.
And no one can tell me that this is the reaction of someone who's well mentally:
Partly to change the subject, Harry said, "Speaking of Dumbledore, have you heard what Skeeter wrote about him?"
"Oh yeah," said Ron at once, "people are talking about it quite a lot. 'Course, if things were different it'd be huge news, Dumbledore being pals with Grindelwald, but now it's just something to laugh about for people who didn't like Dumbledore, and a bit of a slap in the face for everyone who thought he was such a good bloke. I don't know that it's such a bug deal, though. He was really young when they -"
"Our age," said Harry, just as he had retorted to Hermione, and something in his face seemed to decide Ron against pursuing the subject.
A large spider sat in the middle of a frosted web in the brambles. Harry took aim at it with the wand Ron had given him the previous night, which Hermione had since condescended to examine, and had decided was made of blackthorn.
"Engorgio."
"The spider gave a little shiver, bouncing slightly in the web. Harry tried again. This time the spider grew slightly larger.
"Stop that," said Ron sharply, "I'm sorry I said Dumbledore was young, okay?"
Harry Potter and the Deathly Hallows, J.K.Rowling
Like, how can Ron think his best friend is going to punish him by enlarging a spider, knowing he's arachnophobe, and all over a minor disagreement, if he's now comfortable within himself and confident about his place in the world ?
This quote, "Don't worry, it's me, I'm extremely famous", is often quoted by other Ron fans as proof that after the locket, Ron was fine. I never saw it that way, because Ron had already made that kind of self-deprecating joke in the past. I offer another reading of it: it was Ron shielding his children, nephews and niece from unwanted and uncomfortable staring, taking himself as the butt of the joke. And the reason neither Harry, Ginny, or Hermione laughed is because they knew exactly the story that'd gone on behind that joke (I'm sorry to make this funny quote a bit sadder, but well...).
As an aside 1 after that lengthy reply (sorry can't help myself), I'd like to say that you're right when you wrote that Ron always buried his insecurities toward his family's possible preference of Harry in the deepest of his soul because he knew Harry needed his family and wanted him to be part of it, but I'd also like to say that Ron actually makes a lot of effort in general not to let his insecurities "burden" his friends - there's a reason Hermione said that "he always put up with [being put aside]" and "never mention[ed]". There are very few examples of Ron showing that he was jealous of Harry's money/possessions (and they're nearly always brought on by a moment of humiliation), and very few examples of Ron showing Harry he was jealous of his fame and attention.
There are in fact more moments of Ron just not mentioning his problems/bad things that happened to him to focus on someone or something else.
As an aside 2, I'm sorry for all my friends on here, from my now two fandoms, who wondered about my disappearance. Years of fighting against myself have caught up to me in the last months, and well, I've been quite sick. I'll do my best, but I don't want to disappoint anyone if I don't answer or reply right away.