The issue I have with it being a complete stranger is it seems odd to build up enough trust in such a short time(from the timing given by friends of how long Chelsea was not in their presence that evening) that she would have felt comfortable leaving with said stranger. Being a female that made some very questionable decisions in my early 20's and having went to afterparties and such I would only go with friends or people my friends knew well, and I never went alone. I never just met a stranger and went with them all by myself, even if I was wanting a ride. And friends offered her a ride and she refused it has been said. I have seen no mention that there was a sign of a struggle near the party so it is my bet that Chelsea went willingly with the person from the party either thinking she could trust them because she knew them, or she was not alone and in company of others she felt she could trust. I think either this person is in the circle of friends/acquaintances or someone knows something they aren't saying. I just do not believe a young female, especially one who appears to have a good head on her shoulders like Chelsea, would have been that careless to go with a complete and total stranger on her own accord.
Also, I go back to that sketch again. Has the vendor been ruled out as not having any motive to deceive or a possible connection? Could the vendor being leading everyone astray intentionally? Sorry if this has all been discussed, I have been hoping around on these threads and may have missed parts of discussion as I try to catch up.