Perhaps late the night of the 18th, after Mollie finished doing her homework, someone invited her to go to the Denny's with them, down by the TravelCenter near Route 80. If this person was someone she knew and trusted, it would explain her going voluntarily and not taking her wallet or id - expectation was the person taking her was going to pay. And then they would bring her back home...
Perhaps then once she was in the car, it wasn't really a trip to Denny's for a late night bite, but at that point she was trapped in the person's car. Motive? Lust, jealousy?
Presuming the investigators pulled the videos from the Denny's and TravelCenter as well as cell readers on 80. What cars were in that area, went by...
Just speculation on my part, but might explain the search down by Guernsey. Still think the old railroad bed area likely has a hunting cabin/trailer/old shed/mobile home somewhere along those wooded areas...