Mike is making all this happen
Mike has undiscovered/unknown powers of warping reality around him. Because he loves horror movies so much, being in a similar situation triggered for him to subconsciously turn his weekend into one. Even Sally, whose story is told before the weekend happened, was transported from fiction in Mike's head (the video store) to reality (driving the car).
Nick was mind-controlled by the creature to be a slave.
He stays away most of the day - the creature was making him do that. Nick was supposed to come back to find the men dead and the girls impregnated. Nick was supposed to be the creature's servant.