As I understand it, the g540 is basically just a breakout board and four stepper drivers in a single box. Mach3 does all the real work. I am using a CS Labs CSMIO-IP/M where a lot of the processing is off-loaded from Mach3 to the motion controller. Things like spindle control are also handled partly by Mach3 and part by the IP/M which has its own configuration parameters so I'm not sure that any of my system's settings would be relevant and might be misleading. All I can say is that the IP/M and the HY inverter do seem to play well together but I did have to follow the various manuals very carefully to get it all set up. The only thing I forgot to do was change the motor pole parameter in the VFD as the front panel display currently reads half the demanded speed but that's a trivial problem. In your case, there is a non-linearity between input voltage to VFD and VFD speed/frequency which is more puzzling.