I love the idea of 30's Stan fully believing that Bill is an abusive ex-boyfriend and not some interdimensional dream demon.
In a timeline where Stan comes to Ford's aid a little later and Ford is still paranoid but a lot weaker due to sleep deprivation, hunger, general sense or unhealthiness. Stan is trying to pick up the pieces of his brother's constant rambling but nothing is making sense except for the physical and mental shape he is in.
He's as skinny as a twig, he smells horrible, he has bruises and scars all over his body, and he's fucking terrified of something. Stan knows a dying man when he sees one and he is determined to keep his brother far away from that line. So while trying to get his brother into a shower to clean him up, Ford is babbling about Bill, about how he's going to hurt everyone, hurt Stan especially, if Ford doesn't stop him.
But Ford is so unwell that his speech is heavily impaired, and with Stan's physical check (malnutrition, burn marks, scars, cuts, bruises, tattoos that he knows Ford would never get, dirty appearance, sleep deprivation) Stan comes to the conclusion that this "Bill" is Ford's abusive ex boyfriend and that's why Ford called him.
And Stan has every intention to kick that bastard's ass for hurting his brother.
Bill doesn't come that night after Stan had to wrangle Ford to bed. It's only because Ford's body is so weak that Bill wouldn't be able to move it. Come the next morning, with Ford feeling a little better, and now with Stan helping him get something to eat, Ford is finally in enough of a right mind to tell Stan about Bill. Except Stan has already categorized Bill as an abusive ex boyfriend. So every time Ford calls Bill a demon, Stan just thinks that's how Ford is describing him as a person, which makes sense to him.
No matter what Ford says or shows Stan, Stan is so fully grounded in reality that he's able to logically come up with ways of why his brother would be this paranoid and obsessed over a triangle. His time out on the street has given Stan a very unique perspective of life and all the different kinds of people that live. So using his experience with unmedicated mental illness, abusive relationships, PTSD; he can come up with reasons for the lack of sleep, the hallucinations, the physical trauma, the lack of nutrition in Ford.
Ford on the other hand is going insane because, as much as he appreciates his brother for taking a logical path for the first time in his life, NOW IS NOT THE TIME FOR IT! He is terrified of what Bill could possibly do now that Stan is here, but Stan's natural protective resolve about this honestly gives him peace. Stan's outlook on Bill and his actions toward Ford really helps him see just how manipulated he was and how bad the situation was. It solidifies in his head that if Stan really was here before this all went down, he never would have made a deal with Bill.
It also doesn't help that all of Stan's talk of how, "If he ever shows his face here again, I'll show him why it's a bad idea to put his hands on my brother!" and "You deserve so much better than some jackass who just uses you." and, his favorite, "I'm not leaving you alone. Not until I know you're safe." have completely melted through his walls and defenses. Stan is still a knucklehead but in his time of need, Stan came through, and he's staying until Ford "gets better". It's hard to stay angry and bitter when Stan is making sure he eats, making sure he gets rest, making him laugh and smile, making him feel safe for the first time in so long. Old feelings blossom during these trying times and Ford can't help but fall in love with Stan all over again.
Ford ends up seeing Bill again in the mindscape and Bill is taunting, scaring, harassing Ford with images of Stan being hurt by Bill. He says that he will leave Stan alone in exchange for kicking him out and making him go away and to continue working on the portal. The next morning, Ford tries to get Stan to leave. He does everything he can think of to make Stan angry, to make Stan remember that he hates him, to make Stan leave and never come back because the thought of Bill harming Stan the way he has been harmed makes him ill. But unlike canon, while Stan does get angry and mad and ill-tempered, he has enough context to understand that Ford is not safe. And if Ford isn't safe, he's not going anywhere. And he makes that clear as day to Ford that they can throw punches and scream and hit every button, but until Stan knows that Ford is out from under his ex's thumb, he won't go anywhere.
And really, what is Ford to do except to jump into his arms and kiss him? He has lost everything to Bill and yet here Stan stays, tall and unafraid, because he cares and loves Ford enough to put himself between him and Bill. Stan kisses him back and tells him that he's not going anywhere anytime soon.
Bonus Idea:
Bill is getting sick and tired of not being able to get into Ford's mind due to the Ford's foil hat (Stan has tried to persuade him to leave the hat off but after a long and terrifying breakdown, Stan leaves Ford's tin foil hat alone.) So, he decides he's going to fuck around with Stan because he's the one who's been messing up his perfectly constructed plan of galaxy domination. So he enters Stan's mind and after a rather tedious scene of Bill trying to convince Stan that Yes, he is a dream demon, Yes, he is a real being, Yes, Ford has been telling the truth, No, he is not human, Yes, he is the one who's been manipulating and harassing Ford, No, he did not drink the last of the milk out of the fridge (He did), Stan finally believes that Bill is a demon.
Except Stan still believes that Bill is also Ford's abusive ex along with being a dream demon, so Stan is still not scared and even attempts to fight Bill in the mindscape. Like, the moment it clicks that this is the guy who has been hurting his brother, it's on like Saturday Night RAW. He does not give a shit, Bill is a low-life scum to him and he's putting the beating on him. Bill can't even really feel pain but he also can't get a word in either which annoys him, let alone try and manipulate or scare Stan. He ends up fleeing because Bill is getting no where and grumbles that at least Ford let him talk, unlike his brother.
Stan wakes up and tells Ford that he beat up his ex-boyfriend in his mind, also that he was ugly and his bowtie was stupid. Ford is freaking out that Bill made contact with Stan (while trying to afix a tin foil hat to Stan's head) and Stan is living off the high of finally getting to punch the ugly mug who hurt his brother.
