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.
The workaround to install Microsoft Photos Legacy (via Microsoft Photos' settings) is viable solution for the "Not implemented." error message when double clicking on an image file from Windows File Explorer.
Kudos to DamonB23
Anti-Kudos to Microsoft for not fixing Microsoft Photos for the 3-ish months it has been in production. DevOps... when something isn't going right you have the control and responsibility to roll back the changes. Use that power then build better products and try re-releasing! Do SOMETHING to fix your mistake.
The fact that I didn't authorize a new product to be installed on MY machine should be interpreted as an illegal act. The fact that software companies feel they should have the control to dictate which software and versions of software that should be on MY machine have their priorities mis-aligned. I'm for security fixes but not feature updates until *I* expressly approve them.