Sorry cannot help, the GXR is not a computer but I am sure that it can talk to an iMac. The firmware would have Mac and Windows versions.
As far as the mount module is concerned I think they are pretty reliable. As long as they have not been knocked around and sensor/shutter/lens-mount are in good general condition. I have several mount modules and they have all been reliable - I don't propose to sell any of them.
I suggest that the mount module will become regarded as a camera design classic and although camera technology will march ahead the mount module is unlikely to be repeated and will be the technically correct method of mounting MF lenses - especially those of RF type.