Thanks for checking back. In the meantime I was able to complete the update overnight using the USB socket for the external drive, running solely on battery. Part of the problem had been that the 32 GB internal drive was within 2 GB of full even though
I'd removed all of my own files. This seemed to be causing the update to run extremely slowly. I was able to get an additional 2 GB of free space by going through the Windows folder and my AppData/Local folder and identifying everything that looked like
it could be safely removed. I then started the update at bedtime with the battery at 100% and it still had about 30% the next morning with the update completed and working just fine.
Had there not been enough battery for the update, your suggestion of using the ISO would work provided it completed within the time allowed by the battery, or the device could boot off the microSD (haven't tried that yet) so that the computer could be powered
by the USB socket. Too bad it doesn't have wireless charging.
This would have been far easier if update had recognized the 64 GB microSD card in the microSD socket as either an internal or an external drive, but instead it ignored it. Since all other software seems to be happy to treat it as a 64 GB D: drive, this
seems like a bug in Windows Update!