Could he really have been just another long forgotten Elf dragonborn... somehow mistaken or mixed up with the Divine/Aedra?

I understand that most depictions of daedra and divine (like in statues) take on mortal form, but something about his legacy revealed in Dawnguard (what little is shown) seem like he was just an elf (at least, at one time). Could it be that Akatosh is the actual higher/mysterious form of the Time God, and Auriel just an aspect. Maybe similar to Alduin?

It all depends on how you approach the story. Remember that Aldmer believe that they were gods before creation, trapped in mortal form by Lorkhan. To them, rather than be a constant divine, Auri-El would be one of those immortals, trapped as they were, but who managed to escape and rejoin Magnus, who left in the nick of time.

So essentially, we are wrong to use human meanings of Aedra. To the elves, the word literally means "Our Ancestors."

The shot version is, Auri-El is both an mortal elf and an eternal god.

Back when Auriel existed, there was no distinction between gods and mortals for the most part.

Remember too, that the "Aedra" are "Ancestors". The Elhnofey/Aldmer are the closest subgradient to the Aedra.

Elves worship their ancestors as gods, which probably started with the "we believe we are of direct descendants of the Aedra".

To answer, I'm with TengaToppa. He was an elven king and a god. Or more likely an elven god-king-ancestor.

So he was like Trinimac?