We do know that he "survived" for a very long time, he was friends with Galerion, the mages guild founder. So he is very old. Around 1,130 years. He was the first humanoid lich.
That (I think) is what we know is fact (but anything this vague can be changed).
There is more than one Mannimarco.
It could just be that Mannimarco did die (for good) at the Great Battle with Galerion. And when a strong necromancer gains enough power and followers, he labels himself "Mannimarco" and rolls with it. This would explain how it was so damned easy defeat him in OB. In OB you had the combined power of the Arch-Mage (which may or not be the CoC) and Hannibal Traven. So they could have defeated a powerful Necromancer.
It was an Avatar
This would explain why he was so weak in OB. If he was a god, he'd probably be able to send an avatar into Tamriel. I'm not sure about this, but it seems gods are not able to leave their respective realm (wherever that may be). That or they just might be lazy. It seems that instead of effecting something directly, they just kind of sit back and watch, Gods seem kind of lazy like that.
He is dead
This is probably what Beth considers as Canon. He did have considerable power. Making people zombies, even though their still alive. Unheard of in Tamriel, I think.
If I remember right, the only thing that kept the PC from being turned, was the black soul gem with Traven's soul. So it could be that he really was dead. I'm guessing that the PC was a very powerful mage (even though it didn't matter in OB), so its possible he is dead. Or banished, kind of like demons in D&D. If killed in the mortal realm, they would be banished to their home realm for a length of time.
Do you think he is dead? I'm sure lots of people would be mad if they brought him back, but its late, I'm bored, and need something to do.