This is incredible work. I can't write a detailed opinion on each character on this (don't have a couple of extra hours). Amazing review and I see a lot of work with the psychology of the characters. I will make comments about Jeremy, he is not always supportive. On the contrary, he is a rat. Ready to steal a girl from Ian (if your Ian is not decisive enough), then tries to score with Emma at the expense of your success. Also, if Lena has a high lust , he will not mind to have a blowjob from her, knowing that Ian has feelings for her. Jeremy is a bad friend, but you are right that he doesn't reach the chad-status. He tries to get away from his past, where he was shy and unattractive to girls, but he does it dirty.
In regards to Jeremy, it takes a lot of work for him to betray Ian. I actually wanted one playthrough where Ian loved Lena, they were dating, and Lena was fooling around with Jeremy. After many, many tries, I never got it to work, which isn't to say it isn't possible. If Ian tells Jeremy that he really likes Lena, Lena needs to have a big cock fetish, take Molly, and chose to suck off Jeremy after the club to make it happen; even in that case, Jeremy protests and is against it although he does give in. If Lena has more than that one encounter with Jeremy, she will refuse to become a couple with Ian. If she doesn't do anything else, nothing more with Jeremy will occur. Similarly with Alison and Emma. Jeremy will only pursue Alison if Ian tells him he can try, at least until Ian dumps her. He'll only try with Emma if Ian tells him he should. Of course, if Ian has a no-strings-attached or casual relationship with Lena, Jeremy will consider her fair game. That's one reason I referred to him as a "friendly rival." He is only a rival if the player choses to make him one. He more fits the sharing or eskimo-brother role than the stealing-your-girl role. On the other hand, my perspective is limited by my experiences. It is possible there is another side to Jeremy I've never seen, and I don't know because I've never seen it.
Is there a way you can prevent Ian and Lena from breaking up without them both being a saint and only fucking each other? Or is the breakup always happening if they both fuck around?
There are four possible outcomes. Ian can ignore it, they can break up, they can enter an open relationship, or Ian can forgive Lena. The first option is easy, just have Ian not confront Lena about her cheating. The break up option is also pretty easy—I believe it is always available. As for the other two, I don't know for certain why they may not be appearing for you. I believe one of the requirements for them is that Ian cannot still be hung up on Gillian. If you are getting the Gillian dream the morning before the confrontation, you'll need to spend a willpower to get over it. Also, for the open relationship, Lena either needed to decide that she planned on continuing to cheat, or Ian needs to be cheating himself. Hope this helps.