You're talking about Jimmy's boat explosion, right? I guess things moved too fast between finding Madison and the explosion (being spread out in episodes and weeks) that I forgot that. I'm guessing they thought the propeller would chew up the body parts that got loose, but remember in the first episode when someone (Jimmy?) reached underneath the boat to get the weeds and algae? There was some suspense on whether he'd find the remains of the body there... No, like you said, why even bring it back to the island, but if Henry was really interested in Abby, he should have pursued her before even getting engaged or even before getting together with Trish, since it seemed like he had this obsession with her for a long time. I was giving him the benefit of him already knowing Trish, but either way, having the wedding doesn't really make sense since they didn't explain why the island was so important. I think Jimmy was more collateral in the end; Henry didn't expect him to live but since he did, he decided to make use of him and frame him since Shay and Madison escaped.