Thematically, you're doing great, and sorry, I thought this had been changed to EA-
So my bad there. I don't honestly have that much experience with MA, or LA, I'm an EA player, for the most part, atleast that's what I'm used to.
Anyway, I didn't mean that the theme itself was off, just that there are ways to take a theme, and introduce it to the game mechanics-like your great example for "bind scorpion beast".
Magical diversity also isn't necessarily a problem, it just becomes a problem when you've got too much and too powerful. It sounds like you're giving that some consideration, though, and that's good.
Here's a few thoughts:
One way to balance these guys out would be to remove some of their Movement (as in Provinces-per-Turn). Tibetan armour might have been known and renouned as far away as Arabia, but the Tibetans themselves don't seem to have wandered terribly far away from Chomolungma. Not every 3 needs to be a 2, or every 2 reduced to 1, but if you made your horsemen a 2, and your crossbowmen a 1, it would do a lot for balance (and not make them quite as crossbow-crazy as every other MA nation), and also give more reason to use your archers.
Also, your yak-riders are both awesome and terrible. There's really no reason in the world to buy a horseman, with yaks around. But, other than the Gore replacing the Hoof, they're currently interchangeable with horsemen. If it were me (and it's not, I know), I'd take away all of your horse-mounted units' Mountainwalk ability, and just give it to the yaks, which I would then make "No need to Eat" (to better represent the Tibetan treasure-trove that is a yak), and give Stealth (0) to-along with a Gold-hike. You might even put your Rta-Pa on yaks, since that would further single them out from all the other heavy calvalry running around in MA, and give them "No Need to Eat" too-although I don't know that I'd go so far as to give them Stealth. Yak secondform is also a possibility-Some riderless yaks (animals) running around the battlefield would be interesting. I'd give the yaks (only, not the riders) NNE + Supply (2)--giving your army a little food-boost in those hostile wastelands they like to live in--which opens up the possibility of making recruitable yaks. Armies traveling around with herds of yaks would definitely distinguish them from other nations