Here's a bunch of different possibilities I can imagine Adrien discovering the Truth about his father this Season so far! (More might come up in the remaining 11 eps that haven't been released yet)
-Adrien Reads Gabriel's Letter: he finds the letter from "ElToroDePiedra" himself or someone makes him read it (Nathalie/Marinette/someone else?), he reads it and finds out about everything. If someone makes him read it or if he finds it on his own, he might have different reactions. If he's alone he might even not tell anyone, thinking nobody knows about it.
-Nathalie tells Adrien: she's currently opposed to it rn, maybe because of the Kingdom, but she might still decide to rebel and tell him the Truth herself; also to avoid him getting mad at Maribug, directing his anger towards herself instead.
-Adrien accidentally unlocks a secret recording by Gabriel in the PapaCorn plush: while Adrien plays with the frog, a hidden audio message starts playing in which Gabriel confesses he was Monarch, just like in the letter. Could be a recording he made just in case the written one got lost.
-Cerise/Crysalis tells Adrien: she puts her mastermind plan to Expose Ladybug in act, part of which consists in revealing the Truth about Monarch, so she can frame Ladybug as evil and untrustworthy. She might reveal it to the public and/or to Adrien (to hit Marinette) & ChatNoir (to hit Ladybug)
-Ladybug tells Adrien: she reveals everything in a scene that parallels her speech as BugNoire in the London Special (when she went in the cells to rescue Kagami and Adrien), this time telling the Truth instead.
-Ladybug tells ChatNoir: not wanting to keep her partner in the dark anymore (and also because he's next in line of people she trusts, since Alya and Luka both badly reacted to the truth already), she tells ChatNoir about the Truth so they can find a solution together about what they should do, hoping to get a different response from the other two people who discovered it before.
-Marinette Gives Up Her Identity and Tells Him the Truth, Since She Can't Stand Lying to Him Anymore: angsty reveal where Marinette doesn't care about hiding her identity from him anymore, she wants to be fully honest and stop with the Lies altogether, she tells him everything as she's being crushed by guilt and understands how huge the mistake she made is, ready to lose her beloved Adrien forever, considering herself unworthy of him.
-Ladybug & Bunnix take Adrien/ChatNoir in the Burrow to show him what happened: could be combined with the previous two, Ladybug calls Bunnix for a very important mission. They take ChatNoir/Adrien into the Burrow and show him everything that happened in the Season 5 finale and the London Special, while they explain everything with their words. This way he has a 360° view of the situation and he can make his own opinion basing off everything he's seeing.
-Luka Tells Adrien or He does Whatever He Wanted to do with Ladybug's Earrings in "MisterAgreste": Luka discovers the Truth again, in a different way from the first time, but he has a similar reaction, just less panicked. He decides to come back to Paris to tell Adrien whatever he wants to tell him or go ask Marinette for her earrings again, so he can do whatever he wanted to do with them last time he asked.
-LadyChaos tells Adrien/ChatNoir: (if it's actually Maribug) as a part of Crysalis's plan to Expose Ladybug, she maneages to akumatize her in a moment of weakness, giving her a power and a manipulative speech that pushes her to reveal the Truth to the whole city, while she brings chaos with her Ladybug Creation powers, irreparably destroying her own public image and everyone's faith in her. Of course AdriChat hears about it too, he fights her and then erases everyone's memory of the fight (maybe he himself won't forget due to his suit tanking his own attacks)
-Adrien Uses the Kwagatama to Talk to BugNoire or Shadow Noir: I highly doubt this one is gonna actually work, since ShadowNoir technically never existed, as Sass rewinded time at the end of "Ephemeral". BugNoire might not work either cuz Marinette is still alive.. Would she tell him the Truth if he asked to her BugNoire memories alone tho? I'm putting it here anyway, cuz I think he might still use it after discovering the secret in a different way, to ask any Past Cat Holders if they had a Villainous family member and how they dealt with it.
-Adrien Finds the Secret Council's Meeting Room or the Coucil Publicly/Privately Reveals the Truth to Adrien: Adrien explores the mansion alone or follows Nathalie as she goes to partecipate in a meeting. Not Noticing his presence or Thinking he's with them already, the Kigdom members include him in the discussion and he learns everything directly from them, together with some of their evil plans, which he might use to fight against them later.
-Tomoe tells Adrien to "Re-Adjust" him: bothered by Adrien getting in the way, she takes him to the side and tells him everything about Gabriel, to "fix" his behaviour and put him back on the programmed path (him becoming a Shadow of the Diamond together with Nathalie). Giving orders with Rings might be involved.
-Feligami decide to Tell Adrien: Felix and Kagami decide to disobey MariBug and belive it's time to tell Adrien the Truth. At least about the Sentibeing stuff (I do hope it's them who tell him about his nature as a Senti, not anyone else; but idk if they should be the ones to tell him about Monarch, that one should go to Nathalie/MariBug tbh)
Close-Ups:
Do you have any other possible "Monarch Secret Reveal" plotlines in mind? Ones that could actually happen, based off what we've seen so far in s5-6?
I personally think the Truth either gets out in the finale of this Season or in S7, I don't think it's staying hidden for longer than this. There's too many villains who know about it, too much guilt going on with Maribug and one of the people who know is Felix, who might have many reasons to suddently switch sides and tell Adrien about it.
Mah Boi needs to schedule another therapy appointment after this one🗣
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Found this theory video about why it's likely that Adrien will be the one to find out & investigate about the Secret Council + Crysalis's Identity before everyone else!
🍉Link🍉
I think this is how it's gonna go too! It's going to really start moving in season 7+, but now that Adrien's free he can finally do and think whatever he wants
I belive he might even figure out his Monarch's identity not long before Maribug decides to finally tell him (can even be in the same ep: he discovers it at the start of the ep and she tells him towards the middle, so he had a lil time to process the info before she comes in, this way the anger'll be diluted to "you knew and didn't tell me" instead of being one giant world-shattering bomb dropped on him all at once)
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[EDIT] Bonus:
How ✨️some people✨️ think this plotline is gonna go...
another miraculous lb art dump... this might become a usual thing unfortunately,,, I recently discovered harriel btw omg.... they r so cute.... plus psycomedian is one of my fav episodes ever its so funny (excluding the second hand embarrassment lol) I am still redesigning Gabriel designs ik ik its an addiction... I am planning on a Monarbug one too because that design has insane potential!!! like those eyes r so cool (massive sucker for red pupils in character design) okay yada yada yada I am rambling !!!! BYE and dont look at the shadow noir design for too long.. not my best design lulz