So FWIW, an update to this situation.
I flew out to pick up the plane. On the ground the D100 and D120 did not communicate. But during the demo flight, when we took off everything started working normally so I decided to fly it home to Texas. I took off and it still did not work. The D100 did it's thing fine, the D120 did it's thing fine but no communications between them. Also no HSI function on the D100 as the GPS data was not making it to the D100 either. I didn't notice if that worked or not on the demo flight but I didn't want to fly home that way. Plus, the autopilot function and the HS34 did not work either. So back to the airport I went.
We called around and one of the locals recommended an avionics tech who agreed to come look at it the next morning. We stared looking at the network status on the D100 and D120. The D100 could not see any other device on the network. The D120 could see 6 devices, i.e. itself, the HS34, AP74, the 2 servos, and the remote compass. So the D100 was the only device not talking to the others. The harness went from the D100 to the HS34. The tech had a spare HS34 setting around so he fetched it. What a simple fix this could turn out to be, just swap the HS34! Nope. No joy. His HS34 did not fix it.
So we pulled the D100, the HS34, and the D120 and started continuity checks on 2 DSAB network wires between the units. The tech thought he saw a high resistance connection between the D100 and HS34 so he fixed that. Still no joy. So he said he thought the only thing left as the culprit was the D100 itself. Sigh....
I put in a call to Dynon to talk to them. I had to leave a message for a call back. While waiting, I called Stein Air. A super nice tech listened to my story, told me he had never heard of anything similar and recommended that I really needed to talk to Dynon.
Dynon called back and again, a very helpful tech listened to the whole saga. He said that there had been a few cases where the RS485 network comm chip in the legacy units had failed and that it needed to come back to Dynon. The good news is they still repair them on a flat rate charge and warranty the repaired unit for 1 year.
So off to a pack & ship place we went and sent the D100 off to Dynon. Fingers crossed that when it comes back that all is well. We shall see.....