The only evidence in favor of the Lich hypothesis is a very old, very biased account known as http://uesp.net/wiki/Lore:Mannimarco,_King_of_Worms. It reads like something that would get read to little children at night to frighten them. While Mannimarco himself confirms many aspects of the tale, such as knowing Galerion and having been a Psijic, he never seems to have actually been a Lich to me.
http://uesp.net/wiki/Daggerfall:Mannimarco has been cited by many as actually being grim reaperish or skeletal, but upon closer examination the parts of his body which are showing appear to be covered in flesh. Namely, his hands. They are more like the color of a High Elf's hand than of a skeletal hand. His eyes glow eerily, which many cite to be evidence of supernatural lichdom, but may I remind you that this could easily be a magical effect such as Night Eye, for the intimidation effect on any visitors.
We know that Altmer live longer natural lifespans than Dunmer do, and we see several mortal Dunmer sorcerers who have lived to be into their thousands. Baladas and Dratha are very old, and Divayth Fyr is said by some to have been contemporary with Sotha Sil. If that is the case he could have possibly been as old as 4,000! Mannimarco, at most, would have been a little over 1,000 years old at the beginning of Daggerfall. Considering the natural Altmer lifespan is longer than the Dunmer one, it stands to reason that an Altmer sorcerer using similar magicks to a Dunmer sorcerer would extend his life exponentially longer than the Dunmer. Even an Imperial, Janus Hassildor, is believed by many to be as old as he is due to sorcery. The fact is that he's a vampire, but the common populace simply assume he's a powerful sorcerer, and that explains his longevity. Nobody seems to question it. That makes me believe there's some sort of precedent on Tamriel for human sorcerers to live a very long time, perhaps into their low hundreds.
It would appear that necromancy and/or sorcery can exponentially increase any of Tamriel's sentient races' lifespans. I suppose even an Orc or a Khajiit could live to be well over 100 if well versed in certain arts. Granted, Mannimarco has one disadvantage over beings such as the Telvanni - he doesn't live a life of luxury. The guy has been a fugitive, on the run from the authorities for most of his advlt life. On the other hand he's an equally powerful mage to beings such as Divayth Fyr and Dratha. Still, his Altmer heritage must account for some of it.
I just don't believe him to be a Lich. Not because there's evidence that he isn't - you can't prove a negative after all. It's because there's no strong evidence or reason that he is. I think he was always just a mortal sorcerer, except for that part of him that became a god. Powerful, yes. Cunning, certainly. Skilled, without a doubt. But flesh and blood all the same. He reminds me in a way of the character of Darth Nihilus from Knights of the Old Republic II. That being had powerful necromantic abilities, and was thought to be a phantom or a sort of lich by many, and yet when his mask is removed one character remarks "He's just a man. Nothing more." (In Mannimarco's case, just an Elf, but you get the point.)