He killed somebody in order to save his life and the life of another person. This game is nowhere near that stage and there's a massive difference between that and the MC potentially snorting coke. Even on the DIK path, that doesn't suit his character and DPC has stated multiple times that this will not be as dark as AL.
Not die, just a bad OD that she'll recover from. That seems like a much better use of the coke as a plot device than Quinn and the MC snorting it because Riona is the closest person to Quinn and if that were to happen to Riona, then it might just be the wake up call Quinn needs.
And you're overlooking several key things, like the fact that Quinn was reticent to even get it in the first place when Rich asked for it which shows she's uncomfortable in dealing with these harder drugs. We also see her Dad doing it and the woman in Ep 6 who are may or not be her mother is clearly strung out on harder drugs, so all-in-all the game seems to be pointing to Quinn having a problem with these sorts of drugs, therefore it doesn't seem likely she'll use it.
Like I said, people overestimate the extent of the DIK path.
Yes, she was.