Yes, that’s the idea. Thanks for having a look.
Since by default GRBL will not output any PWM duty cycle before running a program (and the X-Controller’s analog output will start at 0V) , the control passes over to M3 Sx and M5 inside the gcode. The manual switch would be an extra safety measure. I’ve seen people running their VFDs with the FOR-DCM permanently shorted.
If someone can tweak their post processor to issue M7 and M9, it’s not harder to do the same with M3 and M5 (if, as you mentioned, their CAM doesn’t do this already).