I think the island needs Sun — but it also needed she and Jin to be apart. I believe she and Jin are, together, a single candidate. But the island didn't want them together so long as non-Locke was gathering up candidates. The more there are to gather, the more difficult it is for him to leave.

Yeah, but why WASN'T it on? Makes no sense. We know from previous seasons that the fence not only kept out the smoke monster: it did serious damage to humans, too. Why were they able to walk right into it? Wouldn't Widmore want that thing on at all times?

Like Sayid, I think non-Locke may have brought her back from the dead. As Zack mentions, she was in a house at the barracks when it exploded and we never really determined her fate. Next time we see her, she's hanging out with her "dad" and behaving oh, so zombie-esque.