Snipped by me. Re: guilt: IMO, JW may have behaved oddly because he felt guilty about what happened that night vs. he was the guilty party. Guilt often accompanies loss under even normal circumstances, i.e., if only I'd done X, Y would still be alive. It's not survivor guilt, it's more about the failure to save.
For instance, if JW had joined LS at SW, she would have probably left with him. Maybe he did hear that she was with CR at SW but decided to let it slide. Or maybe he knew LS was dabbling in drugs and didn't do anything about it. Or maybe they were having relationship issues, which he regrets. There are so many maybes in this case.
I may eat my words, but my guess is that JW wasn't functioning well following Ls' disappearance and was acting irrational due to that. I do think the PI's attitude about JW is interesting, however.