So it seems that as of Skyrim, Bethesda has gone and thrown out the "immortal vampire snakemen" angle of the Battle of Pale Pass, according to http://www.imperial-library.info/content/skyrim-rise-and-fall-blades. The book talks about the Akaviri bending their knees and kneeling before Reman, which sort goes against how the Tsaesci have been described until now: snakes.

We know that the Potentates were definitely Tsaesci, but every text since Oblivion has referred to the Pale Pass warriors as Akaviri, not Tsaesci, and describes them as bipeds, and almost definitely human.

So, on the one hand we've got Tsaesci snake-people who ate/assimilated/replaced the humans of Akavir, and then we've got the Akaviri, who are described as human.

So, did the Pale Pass invasion simply involve Tsaesci and Akaviri humans, or are we looking at a retcon, where snakes are out, and humans are in?

Honestly, I don't even want to think about it any longer lol. I don't think even the devs have decided what they're doing with Akavir.