I think I may have found the problem with my blinky-blink install of WinTen.
I had to run a System File Checker scan to see what exactly was broken, and found there were a couple of files - both with a .json extension - that were unrepairable by SFC.
I do some poking around, and guess what? A Windows Update-issued fix from earlier this year (that was - get this - a Windows Ten PREP file) corrupted the exact same two files, both with the .json extension.
There is a very specific fix that I've found, and (after I save my WinTen install to a USB drive, using Media Creation Tool) I will run it, then do another sfc run to check it.
If it's fixed... I think I'll do another "save" of my system with Macrium, after deleting the old partition, then try the third attempt at installing WinTen - this time from my USB drive.
And THEN... hopefully I will have Windows 10 on this silly machine.