Stupidity: Amy goes into her darkroom and leaves the lock unlocked from the outside of the door so anyone (in this case Adrian) can easily lock her in. Most people going in that room would take the lock with them or lock the door from the inside to prevent this.
Suggested correction: This is a character decision, and a very common one. The house where I used to live had a garage with the same latch type lock. I would always leave the lock on the latch just like Amy did so as not to misplace it. As for your second point about locking it from the inside, doors with a latch lock like this usually do not have a way to lock it from the inside.
Stupidity: Why doesn't Nick just show Darian's parents or maybe even the police the 99 phone messages from her or the shrine underneath his living room? He had had enough of her at this point, and was even willing to move away at that point.
Suggested correction: If you watched the movie you should know that he did tell the police about the shrine. They checked and by that time Adrian removed everything because she's smart.
