I just don't see how it's possible that the two NPCs are one and the same. It seems far more likely that someone decided to dupe a bunch of necromancers into thinking he was Mannimarco by using an artifact he came across.
Re: the battle in Oblivion, I had the same experience Nightbeat. I only had Restoration for magical abilities, as I was mainly a Paladin type. I just waled up to him and bashed him in the face. Power attack, slash+slash.
It was actually enough to make me turn the game off for about three weeks, and even after that I never really got back into it.