Okay I've gone over to the bad side of the viewership. Letty's a plant and nothing on Fishtank runs like how you think it runs. For whatever it means, I want Letty to "win" because Josie is not a good fish anyway.
In short, I don't know exactly how it works mechanically, but either Letty chooses who is eliminated, or if you fight Letty you are eliminated. Letty is rewarded for causing creating chaos.
Some rationale in no particular order:
Letty introduced the baby shaking plan, which some people consider to be the first act of evil in the house.
Letty introduced the slapping game on the night of the elimination and convinced Mauro to slap Vance and Chris later on. When Mauro was getting ready to slap Chris, Ben came upstairs and gave her a cigar with no explanation. Vance once remarked that when things are toxic or stressful in the house, Letty's mood "elevates." Unrelated to this bullet point but during the same conversation he also said he thought she was an actor when they first met - he might not be telling the truth, and instead just trying to shake her into admitting her role? He also asked her who she wanted to be gone next, and she said the freeloaders, not another fish. Vance said that was weird, and she said the other fish don't bother her as much as the freeloaders.
The producers and freeloaders are always naturally oriented against Letty. This could just be a Meg Griffin joke, or it could be a way of giving the other fish a reason to punch down on Letty and justify attacking her. During the fakeout end, Letty was put in a position to properly motivate the other fish against her going forward.
Frank was an enormous asshole towards Letty on his second visit, but on his first he gave her advice and talked about himself and seemed to motivate her to learn from him. Letty said even after his second visit that Frank was one of her favorite freeloaders. After the third or so time he dumped water on her, Frank asked her if she was doing okay, and when she said she was, he said "keep strong, soldier."
Chris seems to waver in and out attacking Letty because he lives there 24/7 but seems to return to it as his "job" most of the time. If Chris had physically assaulted anyone but a plant, it would have been a lawsuit. In fact a lot of things that happen to Letty seem to be lawsuit worthy unless she had consented to them beforehand.
Letty's interviews are always longest, at least it seems to me. Letty also occasionally shares with the other fish that she was given things to talk about by the producers, but not what. Some people said that Letty bringing up the "Josie guro" topic or the "Vance Frontier Psychiatrist/4chan music" topic might have been raised by TTS, but it seems more likely to me that she has topics given to her to use to bother or win over the other fish.
Damiel is for sure the plant, which leaves the open question of why the producer's plant targeted a particular contestant. The most common argument is "Sam hates her by nature" but that's a little anti-fun. Damiel even rounded up the other fish on purpose for a big meeting, specifically to tell the other fish to come at Letty. He makes his exit - his exit as a plant - after threating to strangle Letty and throwing her clothes out. Which is hard to believe he would do to anyone if the contest was meant to be even vaguely impartial towards her.
Ever since Sylvia was eliminated Vance seemed uncharacteristically attached to Josie and aggressive towards Letty, I was convinced he was 1) buying the popularity contest meta and fearing her fans or 2) aggressively bucking Sam's gaslight but I'm thinking more now that 3) he understood Letty was playing for the house and was trying to force a way for himself and Josie to defeat the house. 3 would be stupid of him, but at least it explains why he started to become uncharacteristically single-minded about keeping Josie in the game even though Josie was his strongest opponent. Even outside of the "ignore Letty" strategy he was doing he developed a very personal "stance" about disliking Letty. In his final moments, when Sam said that Vance would have had to do something "out-of-the-box" to save himself like interrupt Letty's interview, a TTS came to Vance in B2 relaying what Sam had just said. He hurried downstairs and seemed to open the basement door just as Letty opened it - bad timing, or did production send her up?
Every fish that was eliminated by procedure and most freeloaders have alogged Letty on social media since leaving the house, even Scott the trainer who basically never interacted with her. Sylvia even after making peace with Letty on her last night has been crazy hostile. Vance "it's just for the game" Latta has also been alogging. Jon is not a good actor and his single-minded attack on Letty during his return was either uncharacteristically focused of him or him failing to hide his instructions.
On Lance's last night he was trading notes with Letty on a scrap piece of paper, afterwards he folded it up and took it home. Production grilled Letty and Vance on what the paper said, Letty refused to say and Vance had no idea. Lance is still the only freeloader who never turned on Letty and has been nice about her on social media.
I think I had more reasons than this but I've been rambling for a while. I still don't have any idea how they would be intending to end the show based on this, but I hardly put it past Sam to rig the entire "game show" to fuck with everyone. It seems like The Cell 2: II has been designed to make Josie increasingly hostile towards Letty - she's started getting physical with Letty during challenges and being more confrontational over time after starting from no relationship at all. So maybe that's part of it. But I don't know.