I see your points. Still, keep in mind, this is a guy who got busted repeatedly for shoplifting (relatively minor items). This is not a Grand Schemer.
On the insurance, most policies require that you "report a loss or injury" pretty much immediately, or they will deny coverage. I'm sure he knew that, at least, which may explain the call two days after she ... went missing.
If he intentionally murdered her, this would be The Big Time for a bit player like him. He would be in way over his head, out of his league, trying to pull off The Big One. The nervousness, the drinking, the sweating, the strange behavior... all fit that mold. He may have targeted the lower end of the island because of knowledge of currents, or precisely because it is more isolated and a better place to Do the Deed. I don't have any problem believing what some have said about his obsession with the NH case; I imagine he may have read a bit about the island, the waters, etc. Introducing themselves at the bar... pouring their drinks from his own concoction in the car...all calculated (weakly) to look like "normal" behavior (in his twisted, desperate mind).
I am looking forward to the moment when LE release information regarding his home computers and other evidence they have gathered. I imagine the picture will become clearer, and I'm betting he's not smart enough to have thoroughly cleaned his home computers (not so easy anyway).