Gate being Osserc is the most far-fetched on our list, however he should be put right there simply by virtue of all other places being already taken by more plausible persons. In fact, all but the Lady/Sister (which are either Menandore and Sukhul or Envy and Spite, i also place my guess on the former) are pretty straight-forward. Gate remains the only possible for Osserc (who sure must be on the list), and besides there is no one else who would fit. IMO raging with wild fire describes Kurald Thyrlan more than it does describe Osserc himself (who up until now has always been cold and restrained), but it's still a hint there.
As for the OD, the theory of Scabandari's soul being placed in the body of the OD most likely had its origins (if i recall correctly) in the bit Fear says about "The dragons spawned a child of indescribable horror to hunt down and take on Scabandari" - which was most popularly thought to refer exactly to the OD.
IMO though there is a little bit of mismatch between the Drift Avalii theory (which has always been REALLY far-fetched, there could be a HORDE of powerful sleeping beings not desirable to be awakened there, and the connection to the Throne of Shadow is not really convincing) and the OD theory. The OD resides in a pocket warren, contained in the Imperial Warren. It has nothing to do with Drift Avalii, so it is sort of impossible that both theories are true
