I feel like the MC getting pulled into Quinn's orbit is inevitable. I don't see him helping her sell/move drugs, or to help with her restaurant, but I do see her somehow manipulating/blackmailing him into assisting her financially, probably after he gets an inheritance from his rich family and she finds out.
Another scenario is that he finds her in a really bad state, either from drugs or maybe she's been beaten up by her dealers, and he helps her get back to the dorm where she breaks down in front of the MC and either we're given a choice to help her or not, or we don't get a choice and the MC says he'll help her out of her mess, but then we get a choice to later on to let Quinn skate or have her face consequences for her actions.
I could also see this whole "break down" thing happen in the blackmail scenario too, but I'd prefer there not to be any any blackmail and that the MC would choose to help because he she's in a rough spot and wants to help her.
I don't think that Jill would be responsible for Bella's marriage breaking up, but that if anything it may have perhaps exacerbated it. For example, if Bella and James were going through a rocky patch, like if he cheated on her but it was a one-time thing and they were going to couples therapy, but things still weren't good between them. Then Jill has a bad date, probably the one she had with Rusty, and she comes rushing to Bella for comfort, the 2 of them end up drinking some wine, and one thing leads to another because they're both feeling very emotional vulnerable. It could have been as little as them making out heavily, but James caught them and then he left. It would be hypocritical of him to take it so hard, but maybe he just needed the excuse to leave.
In a scenario like that, I wouldn't see Jill as being responsible or that it would do anything to ruin this image of her being a nice and fairly moral person. They would both feel bad for what they did, but they'd understand why it happened and I could see them easily forgiving each other and staying friends.