Thats my point. ZC says she was asleep. It may or may not be true, but there is also literally zero information to suggest that she wasn't asleep, zero information to suggest that she was aware lauren was downstairs and zero information to suggest that she intentionally wouldn't let lauren in.
I wasn't suggesting they didn't know who the mystery man was. Lets say the mystery man's name is Bob. I don't think its realistic to believe that JR and MB would decide "Well, Lauren left our apartments of her own free will, walked off, something happened that had nothing to do with us, Bob found her and now she's in trouble. We'll concoct a story about how we watched her leave, but not mention Bob, even more than two years later when we're being sued and are persons of interest in her disappearance and possible death". It doesn't matter if they know this person, unless you're suggesting its someone who has a massive hold over them. All the public attention is focused on JR and CR, and to a lesser (or less public) extent MB and JW. If they had someone else to take the blame, why on earth would they just say nothing?
IMO, the only way I can see your scenario working would be if the 'mystery man' was someone the boys were and are afraid of. And if thats the case, I don't think its as simple as lauren left on her own, mystery dude found her and returned her to 5N... IMO it would be more complicated and nefarious than that.