I don't know exactly how Jacob could get off of the island. That's still yet to be seen. However, It is possible that the Man in Black didn't know that Jacob was off of the island, and perhaps Richard was there to make it seem like Jacob was still on the island. Or, perhaps, the fact that Jacob was still alive, whether on the island or off, that prevents the Man in Black from getting off, and that's why he wanted to kill Jacob so badly.

He found his loophole when it was Ben who moved the island first and got off first, not Locke. I think if Locke had moved the island first, and gotten off first, then if he died and returned to the island, then the Man in Black could take his body, but still wouldn't be able to kill him. But since Ben moved the island, and got off first, he still took Locke's body, but could then use Ben to kill Jacob, as he used the wheel first. I think it the Man in Black planned for Locke to leave, but Ben took the opportunity instead first, and all tho that messed up the Man in Blacks' plan, he did happen to use it to his advantage. Finally being able to kill Jacob.

But, now that Jacob is dead, the Man in Black is able to leave the island. It could be linked with Jacob's life. If he's still alive, the Man in Black couldn't leave. But now that he's dead, he needs to get all of the candidates off of the island too, so that Jacob can't come back. I think that explains that. But I don't know how Jacob gets off of the island tho. He does have a lighthouse, perhaps he has his own boat somewhere. Just a guess. Don't forget, Richard kept getting off of the island too. And I'm not sure if Richard or Jacob used the wheel to move the island to get off of the island. In fact, I doubt that. So I'm thinking, Richard and Jacob used their own boat, which we have yet to see.