I just don't see how it could be him. But I have to admit it would be a pretty dark/awesome twist if the leader of the remaining unturned Ashaman finally came back to the black tower in their moment of greatest desperation only to be revealed as Demandred. Only it makes no sense at all with the rest of Logain's story. Have we ever had a Logain PoV?

Of the three possibilities you mention for Demandred's reveal, I lean toward the third. I just get the feeling we'll find out that there is no way we could have guessed what Demandred has been doing. And that sucks. It is too bad that the one seemingly competent Forsaken's deeds have been entirely off screen in an unguessable manner.