Bottom line: I did the library updates you suggested, and now I have it working. Very many thanks.
The catastrophic failure I mentioned was just a very, very unlucky coincidence: one of the two HDs in my RAID1 array happened to fail while I was doing the tests. I don't know exactly what failed, but the NAS was inoperable until I removed the failed disk. I didn't notice this at first because the NAS box normally lives at the back of a closet. The Zyxel GUI is remarkably unhelpful in this situation, simply showing the whole RAID array as "Inactive" with no options to fix it and no indication that it's one of the two disks that has failed, and it wasn't until I saw the red front panel LED that I realized what was going on.
Anyway, your prod to me to check your example "mount" command was useful in a surprising way...
BTW. Have you tried example from mine post above? Notice the difference between user= and username=
Well.... I thought
I had. (Note, I use "mount" all the time on other boxes, so nothing you showed was any surprise.) But I kept getting "is not a valid block device" errors... UNTIL I took off the trailing slash on the share path. I didn't know that mattered... I guess I must have always "just got it right" in the past, more by luck than judgement.
Once again, thanks for the insights.