So this happened:
As far as I can tell, the Crucial M5500 SSD in that PC (an off-lease Dell Optiplex 760) stopped being a SATA hard drive, although it seems to work OK when jammed in a USB adapter.
So I picked up a new-to-me Optiplex 9020 with Windows 8.1 on an SSD, shrank the partition, tried to install Xubuntu 18.04, fat-fingered the UEFI password dance, reinstalled Windows from the SSD’s recovery partition, and got this display after a while (clicky for more dots):
After letting it stew in its own juices during supper, I forced it off (pushed the power button until it died), restarted, got through the UEFI dance, and it now seems All Good. I made recovery DVDs (remember DVDs?), both before and after the fumbled Xubuntu installation, but didn’t need them.
I expect we’ll never boot Windows 8.1 again, but it’s there Just In Case.