Here's my theory about Michael getting off the island. Back when he got the boat Ben told him to go towards certain coordinates, don't remember what they were exactly, and he will find rescue. So I am thinking that there is another underwater hatch, island that more Others live at, boat, or anything that Michael found and they dropped him off back home. There has to be another way off the island besides the sub that was blown up. If you remember Miles asked Ben for that money and then Locke said there was no bank on the island, Miles responded that he will find a way to get the money. So there still has to be some kind of contact and way to get off the island.