I finally had time to try the "scrub Zune from the system" approach, and it worked!!! On two different computers!
I had already rolled back to version 1803, and tried to disable further updates. (That didn't work.) So I did a full disk backup, then uninstalled the Zune software. Then--and this took some hunting--I found a couple of folders under AppData> Local> Packages> that had Microsoft.Zune in the folder names. I copied the folders into a safe location in Dropbox, and then deleted them.
Then I ran the update to Win version 1909. It worked, and my Username folder was the way it's supposed to be, with no User.000.
Then I did the same thing on my office machine, which had the same issue.
Miracle!
I had spent hours with high-level tech support on this. I can only conclude that Microsoft developers never look at this board, or if they do, they don't care. Every support technician should know about this.
And Microsoft should have fixed it months ago, so it wouldn't happen in the first place.