I think he is a dark elf or one of the ancestors of dark elves.

Why? even though all sources say he is a nord, he acts more elfish.

All of his cultists are elves, specifically dark elves. No Nords follow him. His arrogrance is on par with Neloth and he is more of a spell user. I just get a sense that hes more elf like than nordish.

Well no dur all of his cultists are elves, he can only control people from Solstheim, and its like 90% elf now.

He is a Nord in the game files, thus he is a Nord.

Most definitely a Nord.

Most of his cultists are Dark Elves because 1) The Skaal resist him and Herma-Mora 2) The Dunmer have inhabited Solstheim for quite a long time now.

Miraak being a spellcaster doesn't work against him being a Nord. The Nords were not always so suspicious of the arcane and one of the greatest mages in Tamrielic history is potentially a Nord.

Miraak was born before the Dunmer existed as a people.

oh my god, as if there weren't already enough holes in OP's statement, marvelous