[575] does the 24VDC input actually need 24 VDC if only using the input for homing switches? Can you use 12 VDC? and would it be damaged with a higher voltage than 24 VDC?

Yes, you can use 12VDC for the MAch3 USB controller at the 24V and DCM terminals. I have not tested using 12VDC in conjunction with the ACM and AVI terminals (PWM output) to control the spindle speed. PWM typically requires a reference voltage (from the VFD) and it's own voltage to produce the PWM signal. I am not sure if 12VDC is sufficient for the PWM. I have tested the inputs with 12VDC and they work fine.