But Holly is extraordinarily bad at standing up for herself. Ian could probably have fucked her and then told her to get out of the hotel room because he wanted to sleep alone, and next morning Holly would still tell him it was totally OK after spending the night sleeping on the floor in the hallway. And she’s afraid of telling Lena what happened, so it does lead her into problems, she just doesn’t want to bother Ian about it (see: "bad at standing up for herself"). In addition he only tells her about his involvement with Lena after they’ve had sex, which is convenient for Ian, but not exactly the best way to handle the situation.
I’m having trouble keeping control of all the different paths by now, but I think you’re also mixing up two different paths for Holly. If Holly meets and has sex with Ian at his apartment in chapter 9, that’s because Ian told her the night at the hotel wasn’t a one time thing, which in turn is only available if Ian and Lena aren’t serious about each other. In that scenario I’m pretty certain Lena reacts differently when she finds out what happened at the book fair. If Lena and Ian are serious, I don’t think we have the option to tell Holly it wasn’t a one time thing, and I don’t think Holly and Ian will meet in chapter 9 either. (Correct me if I’m wrong here, I absolutely might be.)
Oh, Lena is absolutely a hypocrite if she’s involved with Jeremy. That’s worse than Ian sleeping with Holly, as Jeremy has been Ian’s friend forever, and he’s also the boyfriend of her friend and room mate. She’s also a hypocrite if she has sex with Mike and dirty talks about him cheating on his girlfriend — considering Lena herself has struggled for a long time because Axel cheated on her. And then when she has sex with Axel literally just hours after having the relationship talk with Ian, and then the events with Seymour a few days after …
But isn’t that a big part of the fun as well, being able to play both Ian and Lena as either nice and faithful or ending up doing shitty things over and over? And while Ian so far can’t have sex with Holly without some consequences to his relationship with Lena, there’s a lot of other options for him where he can act equally shitty and get away with it (for now). Like him having sex with Cindy in her and Wade’s bed, also just hours after the relationship talk with Lena.