Question: Why did Erle try to kill himself?
Question: How can Michael recognize Laurie as his younger sister since he wouldn't have seen her since she was only two years old?
Answer: There is a scene where Laurie dreams about meeting Michael as a young teen. It's unknown whether this is an actual memory of real events, but since nothing indicates otherwise, we could assume the he saw her at an older age when she looked closer to her 17-year-old self.
Question: It is implied that Satan and the forces of evil are always watching out for Damien so that when anybody gets anywhere near to hurting him they invariably meet a very sticky end. So how is it that, at the end of the film, Kate Reynolds is able to stab Damien to death with such apparent ease when all previous efforts to kill him have failed so dismally?
Answer: Damien states at one point that as the birth of the Nazarene gets closer, his strength fades accordingly. Presumably this also applies to any forces assisting him.
Question: What was that first body in the woods that looked like Muffin? It couldn't have been Muffin because she shows up at the end right?
Answer: It never really says what the body is, but I'd assume it were a rabbit or some other small woodland creature. It can't be Muffin because, like you said, Muffin appears later on.
Question: What did Dr. Hirsch mean when he said to Alex how he witnessed "some form of mass neurosis in East Proctor"?
Question: What happens to Harold? We go from him killing a classmate to present times where he works as an intern in a hospital. Surely he'd have been in juvenile hall then prison or in a psych ward.
Answer: It is never explained but the implication is that he has somehow escaped justice for his earlier crime and is using a new identity.
Question: Why does Ann blame Ginny for her mother leaving? She should be angry with her father and Ginny's mother.
Answer: Like all spoiled and mean girls, she blames everyone around her for her problems, when in fact it has nothing to do with her. She wants what she wants, her perfect family with her as the center of attention. The only daughter.
Question: What was the reason why the two divers at the start of the film chose to go down into the sunken boat to strip and make love? Of all places, why do it there?
Answer: He hated the way they were living. Acting human, hiding who they truly were, integrating into "normal" society. He wanted to be wild and free. When they were feared and worshiped.