Your questions are great and you actually sound pretty genuine and amenable to changing your mind on Quinn, which is a nice change from some of the hard line stances people take over the girls in here
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Because Jamie and boop have explained it, I'll also add my thoughts on your specific questions.
Quinn slaps Riona, pimps her out, puts her down by saying her grades couldn't cut it to stay at college, and is basically a terrible friend to her. Despite all that though, I don't think I've ever seen a single person on here claim that Quinn doesn't care about Riona. She does. She lets her smoke their product for free, despite the obvious shit situation they're in, and is clearly looking out for her in the extremely brutal and realistic sense that comes with being involved with drugs and abuse since you were old enough to measure your height with chalk on a wall. Now, you might not subscribe to this explanation of Quinn's behavior at all, and that's perfectly fine. All I'm asking you to do, is to put the really bad actions that Quinn takes during the story, and to contextualize them in another way.
A lot of people who dislike Quinn, maybe most people who dislike her, bring up Quinn slapping Riona as exhibit A for that dislike. That's completely understandable, because if that's how Quinn treats her friends, she can't be a very good person. But the mistake these people are making, in my opinion, is attributing Quinn's slap to malice. It wasn't malice, it was survival. When you're fighting a war, when you're in the trenches, and the guy next to you starts panicking and having doubts, you can't take the risk that they might pop their head up and get their brains blown out. You grab them, and you slap the shit out of them, for their good and yours.
Quinn's fighting a war. She's been fighting a war since she was a kid, as far as we know. Riona's fighting it with her, for reasons we don't know, and we don't know exactly how bad it is, but we do know that she's starting to lose her nerve. Have doubts. Maybe even start to believe she could do well at college and be something else. Quinn can't have that. She's probably seen what happens when people in her business try to get out, especially with an outstanding debt. Quinn needs Riona's head in the fight, for her own good. That's why she slapped her, in my estimation. To remind her things are serious, not out of some sadistic reflex.
That's my own personal view on it anyway, to be read in conjunction with the other explanations of Quinn's behavior from the others. If you can bring yourself to maybe rethink Quinn's motivations for slapping Riona, you might view her other actions in a new light. She really is a great character so it'd be nice if more people took a closer look at her for their own enjoyment of the story at least.