I'm rather enamored by the memory manipulation, especially the first bit with the disappearing extra scouts was nice, and having the PCs regroup each "phase" does allow a clear step in tension. It also makes a convenient narrative justification for keeping the game running smoothly, and I think it works here. Ditto for books and screens, keeps the ball rolling for a one-shot, but feels creepy and scary.
The horror feels horrific, and the whole of the adventure seems like a good one shot!