Well...I don't think crossbows are out of place in MA. And if you *don't* give them crossbows in some form, that'll be a bit of a handicap for them... I'm all for them having crossbows, and if they're non-capital, they would be a great defensive boost for fortresses even with the movement penalty.
And I was thinking more along the lines of yaks not themselves being stealthy, but rather yak+rider equals a unit that can choose the harshest terrain, and then spring out at an enemy, undetected, from a direction they might not have thought possible. More like a stealthy, clever raider, than an invisible yak
The yak by itself wouldn't have any Stealth, and I don't think the elite calvalry should get stealth, even if they're riding a yak--only that specific unit.
As far as NNE goes-my logic was that the rider would be living off the yak, so no need to eat for that rider, other than yak milk, which I understand is very nutritious. Take the rider away, and the yak, since it doesn't have to work so hard carrying the rider around, can then not only feed itself, but produce enough milk to feed 2 other people, instead of just 1.