I get the sneaking suspicion that Jack volunteering for the caretaker position and receiving Reverend Jacob's communion is just a little too neat and tidy for the way this show tends to run. The fact that Jacob inferred that a name being crossed out on the cave is just some chalk on a wall, opens the door to a lot of other possibilities. So I wouldn't bet the mortgage payment just yet, that Jack is a done deal.
I'm also unsure of why MIB's returning to the log flume of light would nuke the island, as well as how Jacob has prevented that from happening, given MIB's amorphous abilities. It seems that MIB/FLocke's newest evil plan is to take one last ride into the log flume of light and blow the island to smithereens (a great band, btw). That sort of implies that Flocke has given up on trying to make it off the island and has instead, decided to become a suicide island bomber.
Or has he?