Alright, I finally read the prologue and caught up on everyone's theories in this topic, I must say that the Stormfather has every right to be a more complex character, but if there is someone impersonating him very few people here have addressed that he speaks directly into Gavilar's mind. The characters that are shown to do that:
The Stormfather (could be himself)
Shardholders (could be any one of them, with their power)
Nightblood (pretty sure its not NB)
Spren to their bonded Radiant (Gavilar wasn't radiant)
the meekers on Patji (yeah, no)
Sja Anat
I think Sja Anat is the most likely candidate if the Stormfather was being impersonated.
She is the unmade that was feared most.
She in interested in being a god in her own right.
She has been in Kholinar (no corrupted spren were seen previous to Oathbringer, but Glys had likely been corrupted there)
@coolsnow7 had said earlier that The Stormfather could possibly have been corrupted by Sja-Anat, I think that is the most likely explanation besides The Stormfather just being The Stormfather. His storming eyes and all the other powers he shows in the prologue would be hard to imitate. Corrupted Stormfather becomes Everstorm anyone?