I think I may have found a fix. Not to the problem, but just a fix to be able to open my photos.
I opened the current Photos app, went to settings within the app window, scrolled down to where it has an option to download and use Photos Legacy and downloaded it. Once it was installed, I clicked on any photo and that asked my what app I wanted to use to open it. I chose the Photos Legacy app and then checked the box at the bottom to make that my default. Seems to be working so far.
I talked to Microsoft three times. Useless.
First, they had me go into Task Explorer and restart Windows Explorer. Worked. Hung up. But the problem reoccurs 17 minutes after the last successful photo viewing (see my repro steps on 11/10/24). Therefore, I got “not implemented” 17 minutes later.
The second time, they right-clicked a photo, selected Choose an app, and chose Photos. Worked. Hung up. 17 minutes later, got “not implemented.”
Third, they installed some updates. Worked. Had him call me back an hour later. Got “not implemented.”
Gave up on Microsoft.
Then I installed Photos Legacy, per DamonB23's post above (11/5/24).
Then I went into Settings --> Default apps, and changed the default for “Photo viewer” to Photos Legacy.
Haven’t had a problem since.