Holllllllllllllllllllllllllllllllllllllllllllllllllllllllyyyyyyyy sheep! That was a frickin' travail.
I have finally managed to get my system drive switched to GPT and successfully installed Win11. It was two-and-a-half days of screwing around with BIOS/UEFI, various disk management programs, attempts at running various command line programs, a sidetrack into a bootable-USB-OS making program called Rufus, multiple brickings of the unit in question, and more.
My virulent antipathy toward MS grows and grows. When one copypastas THEIR command line prompt into one's command line and it barfs up "invalid command," one tends to think ill of the originator.
What eventually fixed it was likely to be utterly inexplicable by anyone not the person(s) who wrote the MS install program: I started a reinstall of Win10, then aborted it and, voila, the computer started working just fine. WTF?
Unfortunately, it is still booting off the old system drive. There's a sparkly new M.2 slot SSD in there now that should be zippity fast, but I cannot convince the computer to boot from that one, despite it holding an allegedly perfect copy of the contents of the current system drive. A project for another day.
Thanks, everybody, for your help -- it is really nice to know I have some knowledgeable friends to help me out of these messes I create!
ETA: Yes, I know I long ago passed the point where it would have been much faster and easier to just nuke everything and start from scratch. It became a contest of wills, okay. I know you've been there, too. (Perhaps I should change my username to Sunkcost Fallacy ....)