After ONE YEAR broken, my wife and I can finally share folders on our desktop computers again, not just in a web browser. I'm glad it's back to normal and fixed, but I am still traumatized by the ONE YEAR of having a broken, unusable product, and not being able to do anything to fix it, except wait for Microsoft to get their act together.
I realized my account was migrating because OneDrive was synchronizing very slowly today, and some of my files I was actively editing today were doing weird things--renamed with my computer's name appended... and even duplicated with my computer's name appended twice, although I was only using a single computer to edit the file, so there should have been no conflicting edits to lead to that symptom. And the more obvious symptom was that the "Internet Shortcuts" which replaced the shared folders 1 year ago got duplicated with my computer name appended. As a result, after cleaning up the mess that OneDrive had made of scrambling my file names and versions on the files I was actively editing...
I deleted and re-created the shortcuts to shared folders within the web interface... and yes, it's back to normal now. Many gigabytes of data are now downloading which should have never disappeared off my computers 1 year ago.
I am going to contact Microsoft customer support to see about getting a refund. But I have no expectation that they will approve it.