On your last point, if I had reached the point of “total desperation or confusion” – AND PROVIDED that Windows is still booting up, reasonably well - I would download the latest Windows IOS file, for the current OS Build version and PC config, from Microsoft, and run the SETUP command, as suggested, from the saved IOS downloaded folder. But, this will ONLY work from a “still working” Windows session, although I think I also did it from a Win-10 Troubleshooting reboot and used Command Prompt option to execute the SETUP option in IOS, since I could not get the OS itself to start properly as required.
Best to do a Full Backup, before such Repair Install (or any other), if possible, just in case.
That is called a “Windows REPAIR INSTALL” – and re-installs the entire OS and Updates, but without touching ANY existing user folders, files, or previously installed user applications. Then you would have a cleaner system to debug further. Far better than a complete Re-install, Restore, or Refresh command, which remove previous user files & apps.
There are desperate times that call for desperate measures, as a clean-up when all else fails to resolve a muddled situation.