For this theory, I'm going to assume the man in the cabin was the MIB.
I was thinking about this earlier, why was the smoke monster roaming around the island, while the MIB (presumably the same entity) stuck in the cabin at the same time.
Now I have no idea WHY the MIB was stuck in the cabin, just that he was.
We can assume that the MIB was Christian (unless Flocke wasn't being totally truthful to Jack, which could be the case). But if he was Christian, then he was the one who told Locke to move the island, and get the whole ball moving with the time traveling.
So if MIB, as Christian, was the one to initiate the time traveling, I assume that he would be able to travel with the lostie's willing.
So we have the MIB, traveling with the losties through time, until Locke goes down into the FDW, and meets up with the MIB (As Christian), who then tells Locke he has to leave, and die, to save his friends.
Once Locke turns the wheel, we get one last flash, and the rest of the Lostie's are stuck in 1974, as is MIB.
Now for what ever reason, he gets stuck in the cabin. That's how the smoke monster was roaming around the island, while he was also stuck in the cabin. There were two of them, one who had time traveled back to the 70's and got stuck in the cabin, and the other who hadn't even traveled through time yet.
Once the ash around the cabin was moved, the MIB, who already time traveled, went to Hydra island to wait for Locke to come back, while the other one, who hadn't traveled yet, came up with the plan that started this whole thing in the first place.
That's it, not sure if this makes any sense at all.
Sure there are probably holes in this, any input would be greatly appreciated.