After what turned out to be an enormous amount of troubleshooting time, the EGR value was replaced on the engine. Also then the engine wiring harness was replaced.
No codes this spring on about ten hours of use.
If we would have replaced all the parts that "Mother Deere, DTAC" suggested in trying to fix this problem I would have been in the very poor, poor house. They were really no help. The local technicians at C&B at Pipestone, MN is who troubleshooted this to the end. A shoutout to them on their good work. I wouldn't kid anybody it cost me a bundle, but C&B also wrote off a substantial boatload of their time.
If nothing else my tractor got to spend a bunch of our cold winter in their heated shop.
Thanks to Jeff and Reed and also to Dwight at C&B Pipestone.