That was my personal mistake, not a mistake that the prevailing theories make.

That was probably done to make Jagar Tharn look scary, but it does spice up the lore to assume there is an in-game reason for it other than the company was trying to make a spooky bad guy. Since his ears are covered, he may or may not have pointed ears. We'll never know.

As with lots of TES lore, it's from the perspective of the author, who is anonymous and writes about two people's speculation on whom or what Jagar Tharn might be. So it's a very grey area. Most likely he is part Altmer and Dunmer, as both elf races are very adept in magic, next to Bretons. He may have Breton blood in him, too.

I've always assumed that Dralsi was Dunmer on both sides, but Karliah's father was not. I'm not sure what race would be need to produce violet eyes. Elizabeth Taylor wasn't a Dunmer, so no help there.

Any race that can have blue eyes, I would think. Nord seems the most likely since Dralsi lived in Skyrim.

Like I specified, the point is that Jagar have been identified as an elf of mixed heritage in no uncertain terms. Yes, the books are historical fiction (at least to some degree. The author's befriending of Barenziah at a later point is curious and I seem to remember she had some evasive dialogue about him), but part of the point of historical fiction is to get as many details right as possible. Mistaking the Imperial Battlemage of Emperor Uriel Septim on something as basic as him being an elf (which is usually easy to identify in TES) seems highly unlikely.

As a sidepoint, the Nibenese had a magistocracy and was the origin culture of the modern "battlemage" in TES, so Nibenese-Imperial ancestry could also explain the affinity for magic. Then again, Shallidor, possibly one of the most powerful psuedo-mortal mages in the Lore, on par with the likes of Mannimarco, was a Nord. So was Azhidal, likely the greatest enchanter Tamriel ever saw. Jagar Tharn's racial background doesn't really matter as far as his magical aptitude is concerned.