Just some thoughts:
Pops has a DC motor driven lathe, humble Myford ML7. The controller is an old Fenner Speedranger, that is a three phase thyristor job but the principles are transferable.
We built a controller that sensed the DC supply to the motor and the control signal to the Speedranger, it was basically a scaled comparator that would drop out the no volt release if it 'saw' excess output, over controller input.
Purely analog.
The reason it was necessary was that one time the 3 Thyristors in the Fenner failed dead short, which is the kind of rare scenario that I understand the op is concerned about.
As for the emergency stop safety switch, an unfortunately named dead man foot bar must be the best choice IMHO for a lathe.
Regards,
Xena.