Aucky - to establish a steady state autorotation at high DA you need to produce sufficient rotor thrust to balance the weight of the aircraft. As you correctly state, the air is thinner so to produce that rotor thrust the RoD must be higher (ie you fall faster to push enough air through the disc).
In this condition, the inflow angles are greater, the AoA of the blades increases and the area of the blade producing autorotative force moves outboard and increases in size - therefore the Nr will increase until you raise the lever to control it.
As for speed, your TAS at 10,000 ft with 55 kts IAS will be about 75kts (using 2 kts per 1000' as a rough rule of thumb) and if that is your VNE in autorotation, you don't want to be faster than that because your Nr will decay (reduction in size of auto section caused by too much AoA and a rearward tilted lift vector).
Your flare effect will be reduced in effectiveness by the DA but it will still work although Nr will be lively as you load the disc as there is less rotor drag at high DA.
Will a high DA EOL be possible? Yes. Will it be more exciting than at SL? Yes.
Aim for a big area as low down the mountain as you can
