I have not done what you want - this is my opinion:
It is possible to have EMC control the spindle as a C axis (indexing, contouring).
It's just not commonly done so there are few example to look at.
The biggest pain (that I can see) is the mechanism for switching EMC from turning mode to C axis mode (spindle as axis) .
There seems to be no standard way yet, but a custom G code can do it.
Others have made two configure files one for using the spindle as a spindle, one for using it as a C axis.
Threading using an encoder will always be better.
Threading using a single sensor is possible (many big irons did it) - the bigger (heavier) the machine the better because the 'flywheel effect' keeps the spindle speed steadier.
Interfacing either to EMC is not particularly difficult - the encoder has more examples.
You didn't mention what interface you will use between computer and lathe nor what kind of motors you will use for the axis.
If your using a parallel port with software stepping and encoder reading, then that limits you a quite bit.