Wow! This is one heck of a theory, Jediphile! It's nice to see you put in so much time to give all those arguments in regards to it.
I also believe this theory is plausible, now that I've read all the arguments. When I first heard of your theory, I didn't really think it possible and I was actually shocked. Although I always thought there was a similarity and connection between the Exile and Nihilus, until now I thought that Nihilus was a Jedi Knight during the Mandalorian Wars, who was captured by Mandalorians and eventually became the force-sucker he was. Your theory puts him in a completely new light. Again I congratulate and applaud all the time and thought you probably put into this!
btw, I've also noticed that under Nihilus' mask there seems to be a face. Actually if you turn the brightness level up on that image of him from Wookieepedia, you can actully see a clearer face, that actually looks rotten and old-like.