I don't see the need for a cover story that accounts for LS until 4:30 if she never makes it to 5 North. If JR has nothing to do with the disappearance other than to cover for some friends, then he must have something HUGE at stake to allow the spotlight to be put on him and his parents on the hook for hundreds of thousand of dollars of legal fees. As for the phone call to DR, he could have said he was asking where LS was, because he thought she was coming back after Kilroy's.
This theory would put the number of people in the know to around 5 +. I don't see that many kids keeping their silence this whole time, especially JR. What does he stand to gain?
In other words, if LS is never at 5 North, then JR & MB are already in the clear--even if they took drugs earlier in the evening, LS very likely did more at Kilroy's or Smallwood (she was alive the whole time, after all). Why would they involve themselves unnecessarily? JR, MB, & CR could just say "I don't know, I thought she was coming back to 5 North but she never showed up. Next day I found out she was missing."