I kind of wonder if these Flashsideways have alot to do with the choices that these characters have to make to get to their destiny, a theory that i think has been mentioned in that island is purgatory thread. But yeah these people guest starring in our Losties' fashforwards, I'm not sure if people have noticed are dead and some that have even died on the island. So possibly MiB/Cerberus is posing as these people witnessing what choice our Lostie is gonna make. Say this is a world where Jacob never influenced, but MiB/Cerberus is still around to manipulate. I dunno.
Although Kate's episode, I'm not quite sure who it could be, if not Claire, if she's even really dead or not, which I doubt at this point being corporeal and all.
In Locke's episode, you could say Helen is encouraging Locke to go see Jack for that consultation.
In Ben's episode, you could either say Alex was the crutch of being the influence or possibly even Locke for getting the whole "you should be the principal" idea in his head.
In Sayid's epsiode, I'd say Keami would be, giving Sayid the choice of showing mercy or death, in which he chose death.
And as for Jack's, I guess you could say Dogen, but his appearance didn't really show any relevancy. Same with Sawyers episode, although Charlotte died on the island, her appearance didn't show that much relevancy either, unless you can count that Sawyer had the choice of not overreacting and kicking her out and could have been more open with her. Like she said, "you blew it"
so i dunno, almost a good theory. thoughts?