Since we're speculating here, here's mine:
I think LS was definitely interested in CR. Perhaps she had a fight with her boyfriend, or maybe the relationship had been on the rocks. I don't know, but I don't think everything was as perfect as the friends are making it out to be. After LS and CR left Kilroy's, they head back to her apartment, probably to get a pair of shoes for her before they head back to her place. In the elevators at Smallwood, they run into a group of the boyfriend's friends. Perhaps they have just left the bar as well, but I have no doubt they were also intoxicated.
The friends are none too happy about seeing the physical intimacy between LS and CR. A fight ensues just outside the elevator, and CR is punched in the face. If I'm LS, at this point I would be furious with JW's friends. I would also want to get the heck out of there, and would take the first opportunity I got to grab CR and run. This would explain why they didn't make it to her apartment and left. At this point LS and CR leave Smallwood. LS is probably very concerned about CR, as he's obviously hurting and acting strange. They stumble down the alley to his apartment. LS is probably preoccupied with helping CR and intoxicated herself, and she loses her keys. I do think they made it back to CR's, and I do think that he went to bed.
What I am sure of at this point, is that LS liked this guy. I feel like people want to make excuses for this, but I think the evidence is clear. She was okay with the "inappropriate attention" and she left with him. She would not have done that if she was uncomfortable and I see no evidence she was forced. I think JW's friends are the ones she wanted to get away from. I also think that there is no way in he** that JW's friends did not make him aware of the situation that night. They felt strongly enough about defending JW that one of them punched a guy. That's pretty serious. I don't see how JW would not have been informed.
I'm not sure what happened after LS left CR's, but I can see the boyfriend coming to look for her and confronting her. He would have been angry and hurt when he found out, and that's not something that you fall asleep knowing. You confront the situation, especially if you're an impulsive, partying college kid. She also would have been angry about the friends' actions. If something did happen with the boyfriend, I think it was accidental. He didn't set out to hurt her, but given the emotions, alcohol and possible drugs, something could have happened. It could be as simple as her heart giving out and the boyfriend, in his clouded state of mind, panicking.This is probably the theory I lean strongest toward.
If the boyfriend was involved, I can see him blaming CR for all of it, thus the confrontation a few days later. An. "OMG, how dare you mess with my girlfriend, if you hadn't done that none of this would have happened" type of thing.
Again, this is all speculation on my part, but none of it is outside the realm of possibility.