In the middle of all of this I explored using the phone's connection to my home wifi, and I finally succeeded. There is a 32TB Synology Network Attached Storage enclosure on the network, which my computers see without a problem. I finally got the phone to see it also, and by logging into it from the phone I was able to download files from it. Unfortunately, it has many thousands of files on it, and the phone can normally transfer only one file at a time. On the phone I'd have to Select All, but that requires scrolling on the phone from the top to the bottom of the 1800+ MP3 files, and that would have taken many hundreds of finger swipes on the phone screen. For lack of a mouse my kingdom was lost. (Sigh.) Eventually I gave up.
But my final success came when it suddenly dawned on me that the external battery enclosure (that wraps around the phone) may have been the culprit. Sure enough, when I peeled it off my USB sticks popped right up on the phone screen. The symptoms of having a dead USB port on the phone were right on; I was just too stupid to realize
why it was dead.

OK, in my defense, the USB port on the phone was still working for taking a charge, so it didn't occur to me that it might be only partially working.