You'll know that Alison goes with Jeremy because Ian, after kissing Emma, will find Alison sitting on Jeremy's lap in the club. That's the first indication. If you play on you will get the confirmation. I don' t want to spoil it but it is not implied at all. It's pretty explicit actually
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Ok I can't give you an accurate answer to this as my memory is foggy. IIRC as long as Alison sleeps with Jeremy the first time, she will continue to hook up with Jeremy even if Ian goes on the dinner date with her and has sex with her. Like you said, Jeremy tells Ian that he has hooked up with Alison more than once. I recall Ian noting to himself that Alison did not get enough from Ian and had to hook up with Jeremy even after their dinner date. So it is inevitable iirc.
Yes you are right about that. Furthermore, during the clubbing event at blaze, if Ian chooses to stay with Alison, Alison further elaborates on that. I recall her saying that Jeremy is a casual fling and she is hoping to end the night with someone else and what not. Again I'm not 100% sure on this as it's been a while. However, if Ian stays with Alison and not Emma at Blaze, Alison stops seeing Jeremy period, at least of chapter 7 so far. Later on, she tells Ian that she has only been seeing Ian since Blaze (this happens when Ian, Perry, Cherry and Alison meet for drinks at Fortress in ch 6).
I believe so. Alison gets very upset and I think Ian does not get to hook up with her the next time they meet. Instead he can choose to spend a willpoint to have sex with Cherry a second time.