But they are the same person whether you like it or not, just because he svckS in Oblivion he is still the same as the one in Daggerfall. and he will be until we get told otherwise, (i havent played Daggerfall but i was also kind of disapointed after i fought him in OB).
Not really. The only proof we have is present in a few books in Oblivion and his own claim to fame. Doesn't really mean anything. From a lore perspective, I could write a bunch of books saying I'm the fusion reincarnation of Napoleon Boneparte, Jim Morrison, Luke Skywalker, and Solid Snake and tell someone trying to kill me the same. It doesn't mean I am.
I'm glad Bethesda was obscure. They probably intended for him to be, but were hesitant, and didn't make irrefutable proof present in Oblivion in case the community (inevitably) found out that Mannimarco is a pushover.