1) I'm not sure what you mean by "There is no reason why you can't just bang and move on, but maybe I was in the minority during puberty." If you're asking why him giving in ruins his confidence to be with Shiori, I kinda get it. Remember, the whole thing kicks off because Maika fake confesses to him to try to get him to confess to her for an easy win and ego boost, but he shoots her down. He's choosing genuine love, grown over years, over lust. It doesn't even occur to him in the moment that Maika was just messing with him. But now that Shiori is in his mind and that he's already told her to wait for him so that he can confess, he's putting his chips down. If he succumbs to his lust, he's not committed enough to be with her. Like if you flip the roles, would you want to be with a girl who shows romantic interest in you but is fucking other dudes while she works up the courage to ask you out?
2) I'm not doubting that's what she represents, but as you mentioned, she's capable of restraint. Her behavior doesn't even come out until the confession game escalates, and she only "participates" in the game because she knows the other girls are just fucking with him. The girls obviously don't have a high opinion of the MC, including Aoi, which is why she thinks he needs saving. She probably just assumed that he'd be romantically interested in her if he saw her as sexually appealing rather than a just a childhood friend. Was it toxic and delusional? Absolutely. But regardless of how unlikely the relationship would have panned out in real life, canonically, she isn't actually abusive in the way you're thinking. The game isn't trying to downplay the dangers of people with this kind of behavior, it's just catering to people's yandere fetishes.
3) Maika is a caricature of conceit, yes. She proposes the game as a means to assert her position as most attractive/popular of the three and the others just go along with it to humor her. She even picks an easy target, a person who's already confessed to her before. No one actually expected the MC to say no. Had he said yes right then and there, she wouldn't have even kept him as whatever the hell he ends up in her ending. But in the epilogue, that's not the kind of person Maika is anymore.
She didn't lack for looks, so it wasn't like she couldn't get a guy. Despite her griping of being single, I got the impression that she didn't want just any guy. If she just cared about grades or status, she didn't specifically need MC to be her friend, so I have to assume she genuinely likes being around him. It's hard to say that she's still a bad person on the inside from her lying about her being her boyfriend. Was it selfish and ultimately pointless? Sure, but I've done worse and I'm sure most other people have as well. I don't think the MC and her would be friends if she didn't balance out her bad traits with good.
I strongly disagree with the opinion that she doesn't care about him as a person. When the moment passed with her friends, she checks on his plans with Shiori and even gives him advice. It doesn't benefit her to do so, but she chose to be kind. If the MC decides to cheat on Shiori with her, she doesn't immediately go with it, she discourages him and ultimately seems happy if he insists on choosing her. If the game intended to make her seem irredeemably bad, they did a terrible job at convincing me.
4) Honestly, I don't think the heroines are depictions of some form of abuse. I think they're just different flavors of femdom: humiliation, semen milking, and yandere. Honestly, I can't see how Rin's issue is fixable, let alone more easily fixable than the other two. I think it's actually worse that she intends to blackmail him with a lie. Either he commits social suicide right now, or hand her the gun to pull the trigger later. How many men do you think she's done that to, to use that so casually? How broken of a person do you have to be to knowingly sabotage another person's relationship?
If you haven't figured it out yet, Rin's issue is that she lacks control in her life and struggles to see herself as more than an object to be used. Her family is in a bad economic condition and she has no prospects for a successful life ahead of her. She might have hung out with Aoi and Maika, but they're not friends. That's why the prospect of MC being her friend and letting her be herself was such a huge thing for her. She didn't trust anyone. She's by far the most insecure of the three because what you see isn't even the real her. Her whole persona at school is a lie to cope with her internal turmoil.
Her changing the least as an adult is actually proof that she's got deeply rooted issues that can't be easily resolved. She's failing at intimacy because she can't trust. She wants to trust that her boyfriend likes her for more than her looks, which is why she doesn't fuck him. But if she trusted him, she wouldn't need to do that. It isn't ironic or out of character, it's meant to show how sad her life is and how despite her friendship with MC, it couldn't fix her.
5) To answer your question, Maika. Specifically, college Maika. I mentioned it in previous posts before, but she seems more compatible with the MC imo. They're in the same classes, regularly hang out together (which even Shiori doesn't do as much), and pays attention to his needs and shortcomings. Even Shiori probably didn't even consider he might not remember to get her a gift.
Edit: Also to add, everyone seems to think that Rin has a thing for corruption, but I'm pretty sure the whole ultimatum thing is motivated by misery loves company. Everyone she coerces goes back to her, but they have this miserable look to them. She's not leading them to depravity, she's trapping them in a state of helplessness.
