I don't see why there couldn't be lore legitimate reasons for a non Alt/Bos-mer/Khajiit to be a member of the AD. On the Summerset isles there are some non-mer/khajiit NPCs who claim their family has been there for generations.

I don't recall Nords in the base game AD zones. You do see the Orc and a tribe of Argonians.

I'm a Khajiit Pact main, so you're not alone in the mismatching. But I don't know of any Nord NPCs that are aligned with the Dominion, just regular citizens, travelers, merchants etc.

Maybe you are a Nord who moved to Elsweyr or Valenwood. It might be harder to explain him living with the High Elves but it's not impossible. If you made him recently, maybe he moved to Summerset.

People are also born all over Tamriel. You could have been born to a Nord mother and an Altmer/Khajiit/Bosmer/Imperial/Orc father easily in the Altmeri Dominion territory. If I were to pick my favorite, it would be an Nord mother and an Ohme father, so your Nord would wear Khajiit tattoos. (The golden First Cat's Pride would be my choice)

If it was an NPC I'd be skeptical. But being the Vestige its baked into the franchise that you can be whatever race you want.

There are plenty of random nords scattered around in different AD towns, but they don't have back stories like the orc you referenced. They just happen to live there I guess.

My headcannon for the reasons to prefer each alliance (and the reasons my toons would give for crossing the racial line) are as follows:

AD: oldest culture, probably closest ties to the gods and best equipped to confront the deadra. Also by far the most personable leader, who you might actually trust with power.
DC: not a democracy but the closest we've got. A curmudgeonly leader, but by virtue of an emphasis on commerce and "success breeds success" you view this alliance as the least prone to brute tyranny and most likely to maintain a functional society.
EP: an emphasis on rugged individuality. Might makes right in this culture, with a nord leader who gained power by virtue of being a warrior. The dark elves similarly view power as an end into itself, going so far as to attempt to steal the power of the gods for mortal purposes. While the chaos of this loose alliance let's a lot of questionable behavior slide, the lack of central authority and the overabundance of ambition provide the ingredients for any future that can be imagined.

I play wood elves in the Pact. I just called them Tiny Nord's. Gave them big beards. They pretty much look like a Nord but were smaller.

Dc alpha alliance 💪

You can always craft a story for your character to justify them being in an odd alliance.

Like I said in another thread, I have an AD Dragonknight who is a Nord and he is loyal to Queen Ayrenn because he feels like the other alliances look down on magic users and he loves the arcane arts. He moved to Auridon a long time ago to further his knowledge of magic and the arcane arts without being an outcast.

I also have a High Elf as my EP main. Pretty simple roleplay here is that she is actually half altmer/half snow elf. (snow elf refugees were looking for help after King Harald incident and found an altmer family, they interbred and she was born). My EP elf has always lived in the Skyrim/EP alliance area and it's all she knows so she fights for Jorunn because she is loyal to her King and homeland. She has nothing against the Aldmeri Dominion and other elves but she does not know them.