Hey, I know this topic's a little older now, but I check http://www.imperial-library.info/content/mannimarco out. To sum up the signifigance of it, according to the Imperial Library, after Mannimarco ascended, Arkay bascially [censored] slapped him back to Nirn, but his physical being there (The necromancer's moon) remainded. So, according to this then, that actually was Mannimarco and he is truly dead. However, there is no scource pointed to this, and I haven't been able to find information like this anywhere else. Could someone verify this?
Still doesn't make sense; the individual you fight in Oblivion has no similarities to the King of Worms.
Considering that the actual KoW was powerful enough in life to match Galerion, and literally breathed death, there's no way that "Mannimarco" was the King of Worms unless being cast back to the mortal realm also reversed his lichdom, and made him into a complete moron to boot.
The only real way for Bethesda to repair it at this point would be to reference that an imposter was killed, or reference some more recent activities of the King of Worms.