Here are some more thoughts on the 3:38 witness account and how it complicates the timeline:
1) 2:51 - LS heads down the alley, presumably with CR.
2) 3:15 - LS leaves CR and MB's place and goes to JR's place.
3) 3:38 - Witness sees LS and mystery man @ 10th and college.
4) 4:15 - Someone, supposedly LS, calls DR.
5) 4:30 - LS leaves JR's place and turns the corner.
We know #1 happens because it is caught on tape. It also seems likely that she does in fact go in 5 north because if she disappears at this point, all of the boys are in on the crime. This seems unlikely relative to the reasonable idea of her going in 5 North.
We obviously don't know about #2, but again, why would JR and potentially JR's guest incriminate themselves if she doesn't in fact come over. Again, the only other option is that everybody is in on it. It also makes sense that she is only in the CR/MB apartment for 20 minutes: CR is passing out, LS is probably recounting the fight story to MB.
If we believe #3 happens, then JR is either wrong about when LS arrives at his apartment (not that we have an official time for this, but it would be the difference of being there for an hour+ and being there for 40 minutes or so), JR is omitting the fact that LS decides to leave at 3:38, he and/or the out of town guest go out to bring her back, or she leaves gets harassed and then returns to JR's place. Either way, the problem for police is that JR admits that he is with her until 4:30, so it perhaps isn't worth focusing on this mystery man. We might think it possible that JR is wrong about when LS ultimately leaves (ie maybe she leaves at 3:30, not 4:30 and runs into the mystery man at 3:38 around the corner). But the DR phone call is the problem: we have a timestamp for the call and if JR states that this call happened before she leaves, then JR is again the last person to see/interact with LS.
In other words, if the 3:38 event is real and accurate, JR is:
1) either the mystery man (or the visitor is) and therefore both lying about the events of the night and still the last person to be with LS. It is certainly worth trying to catch JR in the lie if this is the case, but it doesn't make sense why he would lie about it. I doubt the witness was hiding out of site - if it is JR or the visitor, wouldn't they know that this was seen and that they would need to explain themselves?
2) unaware that LS stumbled to 10th and college before turning back and going back to 5 North and to JR's place. In this case, the mystery man would definitely be a POI as he could have hung around outside the apartment waiting for LS. Not sure why he would wait for an hour not knowing that she would leave again though. Seems unlikely (unless the mystery man is JW, who is outside of JR's place fuming), but who knows. If this is the case, LE needs to get more detail from MB and JR about the timeline and they need to release the sketch of the mystery man.