The first one is that the Tsaesci aren't really serpent-folk in the literal sence. I had a thought that maybe they really are just men, I imagine them to be asian in appearence. Using serpent imagery on many of their flags and such giving them the snake man name. I know the book Mysterious Akavir, says they are golden skinned and all that but maybe it's just a metaphor for something else like the armour they wear. They could even possibly be a race of entirely vampric men giving them fangs, which appear serpent like and immortality as well.
My second theory is that the snow demons are kind of a spin-off of the Oni from japanease folklore, I've seen a few paintings of them depicted in snow. Maybe they are just ogre's or trolls with a little more intelligence than the Tamrielic ones. In the Mysterious akavir book, by them thawing out maybe he means that it gets warm enough for them to invade other races because it is far to cold for them to travel at other times.
Well those are my theories, what do you guys think?