*If*it were scenario two, I really think there were other women who were "hassled" by this man/men that evening. I hate to use this scenario, but think of some loser guy at a bar looking for a "date" as the night goes by. To them, it's a numbers game. 11 women say no, but the 12th says yes...bingo, a win.
Use the same mentality for some miscreant looking for a victim at a party. Because of the time of the early morning hours that Chelsea disappeared, I would doubt Chelsea was the first potential victim the Predator(s) interacted with. SOMEONE ISN'T TALKING/TELLING WHAT THEY KNOW. OTHER WOMEN WOULD HAVE INTERACTTED WITH THE PERPS.
If Chelsea were the intended target, then the Perp(s) would have kept a lower profile, just stalking her.
I think if other women were harassed by her killer you would of heard about it possibly from someone posting on facebook or from LE especially if they looked like the sketch. How many went to this party alone? If some guy came onto them Im sure their friends would of seen it or heard about it..
Whether this person is a multi time killer or this was there first time you have to remember everyone has a first. If the killer was trying to dump her costume in the Flat rock area, then seemed like a poor and maybe rushed moved. Like I said i drove by there and it seems like a dumping ground for people's unwanted stuff.. but there is a path to those buildings. So people do enter that area. Secondly, its by a trailer park/community. If you wanted it gone and stayed gone it could of bent burnt. Bonfires are not uncommon in the area. Put it in a trash bag toss it in some busy restaurant dumpster, some random apartment dumpster. If (and no i havent) i killed someone and was gonna dump some evidence i would drive out of the general area.. to some rural area. Every store, business at one point in the area had CB flyer hanging on their wall. Drive 2-3 hours away. There are Lakes, creeks all over Michigan.. this didnt happen. So maybe this person didnt have a car.. which is why her body and the flat rock location are both off the tracks. Maybe there car was their parents car? Then again how many teens steal "borrow" their parents cars late at night. When I was 19ish my GF at the time would all the time as she lived in different city a few miles away.