Now, the first question is, does she have any clue who the 6 are. Yes, she knows "Kwon" is a candidate, but she doesn't know which one, which means she obviously didn't have that much conversation with Jacob. However, if she knows who "Kwon" is, then she'll probably know who "Locke", "Reyes", "Ford", "Jarrah", and "Shephard" are. She might not know what they look like, and she might not know their first names, but as soon as they inroduce themselves to her, she'll know. She blatantly already knows Jarrah, as she wouldn't have gone through all the trouble of bounty hunting him on to the Ajira plane. She knows that Locke is the bald guy who says he died. She hadn't met Reyes or Shephard until they came to the beach camp and united with Sun, and i'm guessing they would have introduced themselves during that soppy scene. The only one she hasn't met yet is Ford, and considering the fact that a lot of the time he uses a different name, he might be difficult for Illana to find.
Anyway, so she knows their last names at the very least. And she knows that there are 6 left, the 6 being on the wall. But why then, did she say that Lapidus could be a candidate? Well, here's my theory. Bram was wondering why they toook him along. Illana wanted him there because she knew he'd be a useful ally, but Bram wouldn't think the same, so she simply told him that he could be a Candidate. She never really thought he could be one of them. If there was a chance that lapidus could secretly be Ford, then she would have asked him by now. she never thought he was a candidate, but it was the only way she could convince Bram and the Reds to take him along. I am starting to think that Illana and Bram's gang were never really that close. Although being on the same team, it seemed to me like Illana was always somewhat seperate to the rest. So that's my theory on Lapidus being a "Candidate". Concluding, Illana never thought he was a Candidate, she just said that so Bram would let him come.
Now, onwards to my theory about Locke. Illana realises that he is dead when they find the body in the cargo. And she knows that it IS Locke, as Fake Locke introduced himself to the Ajira group AS "John Locke". So surely she must have concluded that if Locke is dead, there'd only be five left? As confirmed by TSM crossing Locke's name out on the wall. This means that unless she DOES mean someone like Lapidus or Richard or someone (and i really hope she doesn't because we've already seen the 6 names, which was one of the most amazing parts of the show so far. They really shouldn't change the facts so late through the show.) Then someone must have replaced Locke, someone important, someone like The Smoke Monster himself? If you think about it, it would explain why he wants to take Locke's form, why, according to Illana, "He's stuck like this now". Maybe he's not "Stuck" per se, maybe he just NEEDS to be in Locke's form so that he can escape the island, because maybe only the Candidates can allow the Smoke Monster off the island. And if he IS a candidate, he can allow HIMSELF off the island...
Well, it sounded good in my head, now i've written it out, it kind of feels wierd. But oh well, feel free to give a simple "you suck" if you need to! :)