I like this theory a lot ! It always bothered me that the Jailer looked more like a prisonner, and you hit the nail on the head with your explanation.
I think the "Jailer" being a usurper is a really interesting idea and twist on the story. I also like this idea because it doesn't change the fact that he is the main bad guy.
We've yet to see what Zovaal did that make the Eternal Ones banish him in the first place, so the possibility is there for Zovaal to be another character all together.
If this theory is true, the mystery remain about the identity of the person pretenting to be the Jailer. He must be a terrible soul, being in the Maw in the first place. It doesn't ressemble any race on worlds we know, like Azeroth, Draenor or Argus. Maybe he is a being from another world, who did terrible things and ended up there. Maybe the same world as the Nathrezim ? An alpha version of a Dreadlord, before any mutation due to fel ?