Regarding the ballscrew hardness. The way that I dealt with my machine (actually an acme lead screw, instead of ballscrew), was to place the lead screw into the 3 jaw chuck on my lathe. Then locked the headstock, to prevent rotation, and used a cut off tool, to scrape a slot in the end of the lead screw. The slot was cut to match key stock which matched an existing slot on the motor coupling (in my case). It could have just as easily been a drive pulley slot (as in your case). The key slot cut into the lead screw was not cut all the way to the end of the lead screw. Which provided a "tub" slot for the key stock to be seated into. Once the motor coupling was assembled onto the lead screw, a transfer punch was used, to mark the center of the grub screw hole into the key stock. Then it was disassembled, and the key stock was drilled at the screw's center, to provide a Secure Grip of the grub screw of the motor coupling, into the detent drilled into the key stock. The key stock being seated into the "tub" slot of the lead screw prevents ANY slippage from occurring end to end! Tip: if you don't have a carbide cut-off tool for cutting the ballscrew, try mounting a Dremel tool with an abrasive wheel into the tool holder of your lathe, and cut the slot with that! It works, no matter how hard the steel is!
Good luck on your build. Looks like you've got it going on!