Your theory seems plausible,
Nazgûl-King. Although I doubt the Mouth of Sauron was involved: the way he was introduced in LotR looks just somehow doesn't fit the picture.
But this thread surely inspires me. I can almost see it all happening before my eyes: the traitor was a Gondorian, something like second-in-command in the fort, and he was envious of the actual commander. Maybe there even was something wrong done to him in the past (like he really should have been the commander, but the other person got the place by some unfair means). Then the Witch-King just managed to bring those feelings to the top so that this poor wronged man agreed to betray his kin. Or maybe he was captured and tortured: maybe something similar to what happened to Maeglin happened to him.
And now I'm thinking this would make a gorgeous RPG...
