I'm gonna be honest and say that I'm glad that Coiny got out, and I wish that he'd left over pin.
Now don't get me wrong, I like Coiny as much of the next person, but his character, at least from a story perspective, was outclassed by the final five by miles and was carried throughout the season because he was a fan favorite. Take bfdia 20, Coiny's whole sabotage and reason for doing so was extremely flawed, dumb, and petty. Like it made no sense for Pin to go back and help him, who had both jiggle boy and needlington weighing him down, because it would put both of them far behind from Book and leave them vulnerable for Fries to knock them off; but lets say hypothetically pin somehow managed to pull Coiny up and kill both Fries and Book, what did Coiny want to happen from their? For Pin to just let him have immunity? Because keep in mind only one person could be safe, and Coiny seems to have no problem "betraying" his allies to win cough bfdia 19 cough*. Even after bfdia 18, he had until then to bfdia 20 to sit down and have a talk with Pin about what happened, but no he had to take out his frustrations with her last minute and cost them both the challenge, which he could have done without putting himself up for elimination cuz, yknow....
HIS ROCKET HAS A F*CKING LASER THE EVAPORATES ANYTHING IT HITS. ONE THAT HE USED MINTUES PRIOR MIND YOU.
I mostly agree with Pin's arguments because like, Coiny has gotten at least 2nd safe in all but one elimination throughout the season, so him going up one more time in bfdia 18 would've put him in no danger, and again, only one person could be safe, so there was no point in trying to help him, especially when book was already close to winning, and like, I would be mad to if someone I considered a friend did that too but 1) I wouldn't hold it against them, and 2) Coiny seemingly didn't care about it in bfdia 19 which could mean two things; Either he actually didn't care about it until some aliens told him that Pin was being mean, or he pretended not to care up until bfdia 20, which is arguably worse, because he gave off the impression to others that everything was fine but had an unnecessary outburst that almost burnt his only friend he had in the game. I honestly think the only reason why people sided with Coiny despite how flawed his argument was is because he was popular, and Pin was hated at the time.
BFDIA 22
I believe that this episode would've been way better with Pin still in the game, sure it would mess up the entire B plot but I feel that the writers could still do something with the WTFers to make it line up with bfdia 23. Pin would've made the main cast of the episode much more dynamic with her bouncing off of both book and leafy, we could've had leafy constantly trying to act buddy buddy with Pin, while she constantly tries to act like she still hates her. This episode could also develop Pin's hatred of book and maybe even lead to character growth, and more unlikely a resolution to the conflict.
BFDIA 23
I know this take is vey controversial, but I really like the final 3 and believe that each of them have a reason to be there and win the season. Book has her underdog status, Needle has her arc of winning for herself and her want to prove herself to everybody for herself. and TB has drastically changed from a shy kind guy to a semi-egotistical genius who uses his smarts to win challenges and who also has a goal of proving that he's not a villain and using his inventions to win (even though I wish this happened and episode earlier). Coiny, honestly didn't really have a reason to be in the final 3 other than he's a fan favorite, "This is Coiny's season" and "He's an obvious winner". One thing that I would like to point out is that, people always talk about how oh "Book's obviously gonna win" or "TB gonna win with no completion" and say it as a bad thing, but then they look at Coiny and praise him for being an obvious winner, which is just plain hypocritical as its only okay for the winner to be predictable if the fan favorite is gonna win. I know some people might object and say that Coiny's "development" with Pin is a reason, but at most it was only a slight hiccup in their relationship, and caused no change in Coiny whatsoever. there is no debate that Coiny breezed through bfdia with no struggle where with book it was an uphill battle or with Needle or TB there is a clear change in there character, where Coiny was a pretty static character throughout the season. I'd say that he was only as good as characters like Fries, bomby, and nickel; not even being as good of a character as post-mechsuit Pin.
Needle, Book, and TB hate
I've been seeing a lot of hate with the current final 3 and saying that it is complete garbage, which I completely disagree with. Most of it is because people are mad that coiny got out, which read the last 4 rants above. I somewhat get why people hate book in final 3 as I can say that she's the weakest of the three, but the hate she gets is somewhat in bad faith. People constantly bring up how she's bad in challenges, which is somewhat true but fails to consider that the competition she is up against is fierce, and also that she still puts in effort to win despite what some people claim and she often came close to winning. Another thing that I don't see mentioned is that, along with coiny, book can easily be put up for elimination and not have to worry about book getting out, which is why the writers put her up so much so that other characters that the plan to write future episodes with can stay. The other 2 I don't get the hate for, because its mostly them saying "Oh I don't like *insert character*) which is a personal opinion that doesn't speak about the actual quality of the it. I've even seen some people say that its worse than the bfdi final 3 which, one the final 3 didn't even get to compete in a challenge or have any competitiveness, and 2, the final 3 were basically all friends and showed no real emotion or conflict( I know that they were written back when Cary and Michael were kids and I don't expect top of the peak level writing but still, crazy how people unironically say this is objectively better). People have also been brining up arguments that they were bad/ did nothing during pre-hiatus, which is a really bad faith criticism cuz THAT WAS WRITTEN A DECADE AGO BY TEENS. Needle was still a joke character TB had the misfortune on being and the extremely large team, leaving him little screentime along with being eaten and presumably dying in evil leafy, and book, again, was on the giant team for four episodes before joining freesmart and becoming more of a character. All of the discussion of how the final 3 could be better is just people saying *insert character(s)* bad and putting in their presumable top 3 in bfida in with no reason or very weak ones for why they should be in the final 3. I've seen people unironically say that nickel and bomby should be in the top three, which would just be bad because "being funny/silly" is not a valid reason to be in the final three, and definitely not surviving over a complex and well developed character.
Voting discourse
to sum things up many people say that coiny got "robbed" and a smaller margin say that it was rigged (which is plain wrong). People are calling the voters dumb for "voting off all the good characters" which is stupid cuz I'd argue that since rocky got the boot that everyone in the cast ranged from good to great (with the exception of bomby for being okay but I digress and its not really that relevant). I feel that many people are taking their favorite getting to the final four for granted, which is a huge achievement from being almost the 3rd boot, only staying in the game cuz the votes were rigged by outside forces. Like, people act like there is no reason to hate/dislike coiny, and that he could do no wrong, and that he is the main character of the season, but there are many.
- He's only in the game because of the votes being rigged (unfair and petty but you get the point)
- people could seem them as overhyped and overrated
- his argument of Pin and him being in the wrong
- him almost killing woody
- he always pesters needle about being a woah buncher although she made it clear that she doesn't want to deal with that team anymore
- They think he's undeserving of the final 3
- people want needle and/or book to stay more
- people just don't like him, plain and simple
Side note: its crazy how people constantly say that book is just sympathy bait and is only here because of sympathy voters(which isn't even that feasible because its vote to eliminate) but then there's scene like Pin's """"betrayal"""" in bfdia 18, the cliffhanger of bfdia 13 and the scene of Pin's corpse in lave in bfdia 22, which would make the viewers much more sympathetic to coiny than it would to see book die and lose a bunch, which I've seen make her more hated if anything. I want to clarify that I don't believe that the writers are trying to sympathy bait with any character and this term is just souring character discussion.
That's mostly it tl:dr, I think coiny is overrated(not a bad character) the discourse about the elimination and the final 3 is way overblown, and that pin should've stayed over coiny

I don't completely disagree with most of your points (I had voted for Pin to stay over Coiny too), but I think the section about Coiny in BFDIA18-20 kinda doesn't understand what the point of his conflict with Pin was all about.
Objectively, both of them were in the wrong in different ways, and the issue was that both of them just assumed the other should have been okay with whatever they wanted. Coiny specifically was indeed in the wrong for assuming him helping her should have been transactional, and thinking more with his heart than his brain. Also, Coiny was, indeed, hiding his true negative feelings on the event until the end of BFDIA20, but not in a malicious way.
The thing about Coiny is that he likes to appear as confident and unbothered, so he just pushes his feelings down and if he needs to, sulks in a corner for a little bit (weeks). Leafy had already clocked him when she prodded him on whether he was really as over it as he made it out to be.\ He hides those negative feelings not only from others but from himself, he doesn't like to think about what hurts him. At least up until BFDIA21, when he realises his little showy outburst ruined the closest friendship he had left, and apologises about it.
I don't think Coiny is as uninteresting or plain as you made it out to be, and I do think him surviving this elimination could have brought up more for his character, especially because, as much as I like his friendship with Pin, it ends up absorbing a lot of his character. I think he really needed a chance to play on his own without Pin haunting his narrative, but BFDIA chose to never give to him, since BFDIA22 was a mediocre episode for him and it ended up with him being stuck with her again. I love both of their characters, but it's unfair that Pin gets to have her own friendships, her own development, while Coiny is stuck playing sidekick when he has so much to offer.
However, again, I agree with most of the rest of your post. The hate towards the final three is mostly unwarranted and all three of them made good use of the screentime they were given to become really interesting characters (I'd say TB and Needle especially finally got a proper chance to shine as themselves, and it was great). Would I have liked Coiny to be there as well? Yes, no point hiding it, before BFDIA23 and against all possibilities I hoped Book to be out instead. Am I angry he's out? Not really, although I hope he'll still get his chances at a narrative without Pin one day.
the hate for the final 3 is forced i feel like it's easily the best out of the entire series. all of the charactershave their own motives and bounce off each other extremely well
absolutely agree. something in my opinion is that I feel like..what would coiny really be able to do if he was safe? now it goes the same for needle, she didnt exactly do much..but I feel like the writers were done with him. And its sad, because so many characters were eliminated due to their lack of relevance anymore, or their "arcs" being over. And it happens to so many of them, that I feel like coiny simply got out because he didnt really take up much room. his story was over, and what was he gonna do that whole episode in the lava?
I just feel like, despite not being the biggest fan of coiny, he falls under the same effect of arcs being over, and he didnt deserve it. Last episode, he was a bit bland, and a little rude, but that may have just been caused by him not being able to interact with pin anymore. There are characters that so many of them rely on for relevance, and with those gone, they will be bland, and eventually leave too. That's kinda why I like TB a bit more now. Sure, his feud with GB showed that he was playing for both of them in a way, but he had changed because he didnt want to be all nice and boring, he wanted character. And so, even with GB eliminated, he does a good job at being his own person. (not that I liked him becoming "evil", that was kinda sudden). Also, I do want to point out, that feud was very odd, GB has a whole incinerator, and shes the one who calls him out? Anyway, sorry for the yap :(