I did look like a pretty un-survivable explosion, but I also thought that about the hatch explosion and it didn't end up killing anyone.

Also, I think Michael was trying to kill himself only when he saw himself as a miserable human being who didn't deserve redemption, and didn't have a purpose. I think maybe his perception changed. Michael had found a purpose toward redemption--he was determined to sacrifice his life to save his old friends. I think Christian's message can be interpreted as "You've done all you can do, you've earned a second chance, now get the hell off the boat."

I don't know if the island really needs to communicate to Michael that he can now die. Mike didn't need that information, but he may have needed a prompting to drop one action and go on to another task, maybe. Perhaps the island has another job for him.

I had another interesting thought: Did Michael stay behind because he thought he couldn't be killed?

And we need to remember: If the island doesn't want you dead, I think it is powerful enough to save a character from anything.