I did, indeed, wonder whether the connection was bad. I checked at the
computer and printer. Everything was tight. I use a short USB cable to extend
the adapter's cable, which otherwise wouldn't be long enough. While monitoring
the printer under Devices & Printers, I brought the connection at this point,
and got an immediate "Printer Offline" response. This confirmed that the
computer and printer were communicating, on at least the control level.

That should be "BROKE the connection at this point". Ouch.