What are the pro's and cons' of the PTFE nut that you suggest?
PTFE will eventually breakdown but it is quick and easy to use.

As far as i know commercial machines are metal nuts.
If you were making a commercial unit then it would need to last longer - bronze half nut would outlast any plastic.

I don't think you'll need a half-nut (although it would not be difficult to make)

Back to the leadscrew surely despite what you say the shorter the lead of the screw the better for my application?
Otherwise why make such a range of leads? And whats the purpose of a 0.050" lead screw? or why would you want that over a longer lead with a slower drive? etc
50 thou" pitch is a simple 20 TPI that can be cut on almost any screw cutting lathe.

The reason for the proliferation of leads is to match the screw to the motor, drive and required speed. For a lathe that's only creeping along a fine pitch would be my choice.

BTW if you have a manual or other details of a VMS 70 then pinch as much of the design as you can (why re-invent the wheel :))

since the signal monitoring and algorithm required is going to be one of the more difficult features to design
I don't think vari-pitch is particularly complicated. It just needs a delay line (or a Studer with preview heads) to give the lathe time to open the groove before a loud passage arrives.

I used to make up U-matic digital master tapes with, if memory serves, a 300mS mono preview on the analogue tracks, this was back in the days before we had digital delay lines for the cutter preview.