I think the fact that they recently had Hurley point them out and say, "Maybe we went back in time and that's us" lets us know that the writers are aware of our theory that Adam & Eve are two of our Losties--and they have something else planned.
I think that it's far more likely that the time travel is behind us, and we will discover that the most obvious answer is correct--the bodies belong to previous inhabitants of the Island. Maybe the bodies belong to MIB and his crazy mom. That's my guess. Now MIB has to go around assuming the form of others because his physical body was taken away by Jacob. Add to that the fact that it would make sense for MIB's body to have those little black and white stones, since we are seeing that they are a part of MIB and Jacob's ongoing "game."
As I understand things, it is correct that the flashes took them back to some point prior to 1867. Locke goes down the well, and then there is a flash that seals the ground. They turn around and see the statue, while Locke is underground with the wheel. Locke then encounters Christian (presumably MIB?) underground at the same time. So why was Christian appearing on the Island pre-1867? Or was MIB flashing along with our characters? If MIB was waiting for Locke in 1867, then I cannot explain his appearance and comments about saying hello to his son. (This would chronologically have been MIB's first encounter with Locke.) All they needed was one more flash that would have removed any timeframe/context from Christian's appearance in the cave--but as it stands, we saw Christian on the Island some time before the arrival of the Black Rock. I can't explain that one yet...
(Unless MIB was indeed taking part in the flashes, and possibly even controlling them and bringing certain people along in order to put the final pieces of his plan in place.)