I'm using an external source for all 5 volt power, and I checked with my Fluke, all voltages rock solid at 5v and 7.2v.
Can I assume that the 28/29 pins remain the same for the drive motors regardless of which mode the radio is in?
There may yet be a blown tx/rx chip on the 2560, if the tx signals aren't getting through the Arduino. I won't know until the new board arrives.