As for what soil it's in, it looks like a mix of bentonite clay (hard grey kitty litter?), with haydite, and rock grit. Weirdly, the particle size on top is the largest, and particle size gets smaller towards the bottom, maybe because of breakdown of the clay. It drains very fast.
As for why it's looking on the dry side, the pictures were actually taken after it had been just watered. After watering you could hear the crackle of air, but visually it was slower draining than usual. But really if you put your hand on the soil and try to loosen the top, it's all frozen, a solid brick.
As for maintaining humidity in the wintering area. The area is three sides brick, one side poly sheeting, about the size of a closet. Air gets in from the poly-sheeting side. The humidity outside is probably 20-30%, and if I were to take a guess, not much better in the wintering location. Maybe someone has some suggestions on increasing humidity? This factor would probably be much easier if I had a greenhouse.
I really don't expect much from the tree, I think it's not quite dead but it might be on its way. I'll still give it some TLC to see what it does in Spring. The real stress right now is making sure I don't lose my other trees.
After posting I did a careful check of my other trees and there are some branches that concern me. It looks quite different than the Nishiki, but as seen in the picture, a couple buds have lost color and have turned to a green-grey. The Black Pine pictured is in a 50/50 akadama/river rock mixture. I put this one in a larger box and actually mulched it up to its first branch, in addition to putting it in the wintering area. What do you guys think?