It's less of a question of predicting whether he was transmigrated, and more of an issue of Osan'gar's death being in a less permanent fashion both the first and the second time around then some of the other Forsaken. Bel'al & Semirhage, for example were killed by balefire on-screen with us seeing their deaths, while Sammael's soul was eaten by mashadar, so there is no question as to whether their deaths were permanent.

Lanfear, Osan'gar and Ishamael, though, "died" in ways that leave whether they were defeated up in the air.

Anyway, just a minor clarification - I don't have any theories as to who died, why they did and whether they were transmigrated.