Well, I got some time to catch up on this thread . . . . interesting new ideas presented . . . BUT
We old engineers have a saying "you can't polish a turd"
All of the problems are the result of relying on friction alone to transmit torque. That's the turd.
My original solution is still the best, methinks. Positive drive = end of problem. Easily accomplished with a dog drive on the end of the spindle as I mentioned previously, and since I now see that the plan was to make new tool holders anyway . . zero added cost except for the dog for the spindle nose, which could be a simple bolt-on.
Ultimately, I would prefer Vmax' solution of a new spindle (although I would build it differently) as a rugged and permanent high performance solution, but the current scheme could be made to function, I believe.