"If it is gets stuck at firmware version I would suspect either a defective si5351 clock generator or an issue with the i2c bus (A4/A5 from memory). Check for shorts, and if you have a scope check that you get a 3.3v swing on the two i2c signal wires."
My scope seems to be acting up and hasn't been very helpful lately. I do get ~3.2v on a DVM AC reading on the A4/A5 pins of the Nano itself.
I also found the Raduino clk2 pin grounded out to pin 4 next to it. It fails to explain a similar failure on the backup Raduino board unless I fried 4 other Nanos first (which can't be ruled out until we know). So, while I have two brand new Nanos in packages I could build and put together, I have no further Raduino boards to try except for the one that's built into the other radio that I don't want to tear apart.
Maybe it's time to get a new spare Raduino that has not been put in a questionable circuit since repaired. Would anyone care to recommend the best aftermarket Raduino clone?