Why not hang an encoder directly on the spindle, that takes the motor out of the equation. That's what I have on my lathe. For threading I just electronically gear the Z axis to the spindle encoder, and pick up the index pulse to time the engagement. Since CamSoft is most likely using a Galil controller, this becomes really easy.

Don't worry about spindle speed control, it doesn't have to be exact. Just sending the analog command voltage without any feedback is normally fine when using modern VFDs, especially in sensorless vector mode.