I'm torn between these two scenarios:
If JW was graduating or even just some of her friends graduating, it could have been an emotional period for LS. I was emotional and behaving irrationally the last weekend with the friends who graduated the year before I did. It was the end of something, and I did not want the parties to end, but at the same time, was trying to distance myself from those who were leaving and convince myself I would have just as much fun with the people still around.
so...let's say JW said he'd come over after the game. Game ended, he didn't come, LS feels hurt/emotional about it, decides to go out with some of her guy friends. HT does not approve, out of concern for LS and loyalty to JW, and expresses that, to LS's annoyance.
LS goes out with the guys. She does drugs she does not normally do (part of that behaving out of character, trying to bond with the people who will still be around next year). At the bar, she sees some of JW's friends, who (like HT) show their disapproval of her party mates. She gets out in a hurry, leaving her shoes and phone behind. JW's friends want to know where she is going, so they go back to her apartment. They get there faster because she is walking barefoot (and maybe went a different way). at the apartment building, the altercation makes her think that her roommate was somehow involved in alerting JW or his friends to her going out with the guys, so she leaves angry at her roommate instead of doing the logical thing and staying home. She walks CR home. His roommate gets him to bed, and she is like a lost puppy, not really sure where to go but not wanting to be alone. She asks him if he wants to party, he says no, she says she will go find JR. but after leaving MB's apartment, she has a change of heart. What she really wants is JW. At this point, she either heads for his house, or for the bar where she left her phone, because she is still a little pouty that he did not come over right after the game, and she wants to see if he got in touch with her, in which case she'll go to his house. She encounters the dark-skinned man, who works locally and is walking from the bar he works at to his car. Either before or after getting to the bar. He offers to get her home. But at this point, she is ready to pass out and is locked out of her apartment, and does not want to call her roommate because she is still a little mad at her. So she gives him the address of the closest trusted friend, JR. He gets her there, continues on to his car. Cameras show him walking alone after the 3:38 encounter, so LE clears him. LS is now back at JR's after the 3:38 encounter. Now, either she is still trying to find a place to settle for the night, but does not want to call either JW (who she thinks blew her off earlier and/or is mad at her for being out with the guys) or her roommate (who she thinks ratted her out) so she calls DR, who doesn't answer, then decides to walk home or to JW's despite her reservations about the roommate or JW. Once outside, somebody, maybe the really angry friend of JW's who hit CR, maybe a stranger, maybe even the man who saw her earlier while on foot, is now waiting in a vehicle. Afterwards, none of her friends are sure what happened, but are afraid to admit to things they did that contributed. HT does not want anyone to know that she and LS had a disagreement or that she maybe let JW and friends know what LS was doing, because those things contributed to the tragedy. JW is afraid, but not sure, that his friends did something to his girlfriend on his behalf (for cheating) or that something horrible happened to her while she was cheating. JR's memories are vague, because he was under the influence, but he does not want to admit that, because it leaves too much room for hypotheticals in which he killed her.
Alternatively, while at JR's, her heart stopped, he panicked, called DR because DR supplied the drugs that the three of them did together before going to the bar, and it would seem logical to find out if he knew anything about the drugs. The question here is how they would have gotten her body away where nobody has found it yet.