Couple more thoughts:
Making a spindle from scratch: That comment was made to Vmax who apparently has the capability to machine a spindle, and who also presented the idea of someone making a custom spindle as a product. Many moons ago when I had my own small shop for building prototypes and specialized high strength parts, I did my own heat treating and had a tool post grinder, so like anything else, no big deal if you have the tools for it.
That said, I don't think the spindle is, or should be hardened, except perhaps case hardened to a few tens for durability. Spindles are subject to shock loads from chatter and hardened steel is not a good choice for shock loading, unless you are talking about S7, which IS expensive, is precisely hardenable to a specific depth, but you would still leave the core soft and you would still have to be stress relieved after hardening and before grinding.
I presume you can still buy pre-hardened and stress relieved machinable rounds? I used a lot of this stuff before I got my own furnace. However, I think for the purposes we are talking about here, a new spindle made from stressproof steel round with no heat treatment at all would be adequate. I would grind only the R8 taper and then only after the bearings were installed and it was in the head already. I replaced the pathetic bearings in my X2 head and then re-ground the R8 taper and I show zero runout on a .0005 reading Starret, which is more than adequate for my purposes.
Positive drive: Auto-changing an R8 is frought with challenges, but it seems to me that the Tormach setup could be easily modified for positive drive, potentially halving the tension requirement on the drawbar.