No longer able to program my NUCLEO-H743ZI
Following up. I finally decided to buy a new NUCLEO board (heck, they're remarkably inexpensive for what they are).
Yup. Code loading, running. Seems that all is well.
Now if I only knew what exactly I did to fry the old one so I could avoid that mistake again...
In any case, hh, many thanks for your time on tracking all this down.