I imagine the wreck was probably exactly what the creature had in mind--it's clear he is a chaotic influence, almost elemental, something morally ambiguous. Our hero was definitely not in his right mind. He was in turmoil------the constant crashing of the sea and thunder as he narrates are indicative of his mental state. And witnessing the odd, eerie spectacle of the shipwreck could almost be viewed as the wreck his life has become.

In regard to the treasure, a big part of the moral to the story, metaphorically speaking, is that Juliette and her love are the real treasure, not the box of jewels. He learns that obsession with material possesssions, and the pride that comes with that obsession, are a negative influence on his life--- so the treasure becomes a bad thing.Something to be avoided. He actually ends up recieving the true treasure in the form of his marriage to Juliette. Also, on a purely literal level, I imagine the whole experience was so horrific to him that he just didn't want to have anything to do with that chest and whatever evil influences might still hover around it. It's better to just leave some things lie.

At the end, I don't believe the three characters' reactions are meant to be explanations at all. Rather, they present alternate ways of viewing such a tale. Different people respond in different ways to frightening, painful situations in life. Not any one way is the absolute correct way: Some, as Juliette, just write such things off and deny them. Some see them as an evil influence, something negative. And some (perhaps the oldest and wisest) see the positive effects of even the darkest situations, taking away a valuable lesson and counting it as a blessing. These last two are sort of a "glass-half-empty / glass-half-full" scenario. So there's not really a right answer, but choices. Which one is right? At the end, I think the narrator makes the healthiest and most positive choice, crediting it as personal redemption brought about by a guardian angel.

And the two wounds, and no body to be found? Well, perhaps the creature was the narrator's own dark and ugly inner self--the jealous, greedy thing hidden behind his handsome face which he struggles with and ultimately conquers. (Another example of his dark half is when he nearly killed the other boy during his childhood.) If the creature caused the wreck, maybe, on another level, the dark half of our hero's personality made a wreck of his own life--a wreck which he, himself, must clean up. Taking responsibility for one's own actions is the main pillar of existentialism, and a philosophy with which, I imagine, Mary Shelly would have been very familiar. Such themes run through Frankenstein as well.