There's no end to detective stories where the real culprit is actually the one who called the cops or who seemed to be helping the cops the most - only in order to lead the investigation from the shadows in a way that suits him/her. Let's not forget that the Classics club was originally called to save the movie by figuring out Hongou's incomplete script. So, to be specific, already at that point a more suspicious man could have thought where the movie abruptly ended wasn't according to the original script. After that the situation only got worse for Oreki. He was guilty of forgetting half of the case by only finishing the script and not honestly figuring out Hongou's intentions. That was his own fault, plain and simple, under manipulation or not.