Exactly, so i think over 200 years of history it is possible the Mages Guild ban could be rescinded and therefore Necromancers may be apart of the guild itself
The thing is... Necromancy is one of those "icky-feeling" things. Raising the corpse of someone's loved one to fight for you just seems wrong, so I can see why it's considered an evil magic, and it would make sense for the guild to dislike it.
I do, however, think Bethesda should semi-retcon the death of Mannimarco. The obvious route would be the whole "that was just one of Mannimarco's many avatars" explanation that a lot of people believe anyway. After all, it didn't look like him at all, nor was it particularly powerful. Bring him back and give him the fight he deserves.