Update 6/9/20 (final update): Tried re-installing OMV5 on my backup server and none of these problems appeared. So that's a clue. Did another fresh install of OMV5 on my production server and boom! all my difficulties vanished. Can't explain it. Just updating this note in case someone runs into similar problems. Wish I could give a more definitive answer as to what was fixed by starting over.
Update 6/7/20: Turns out the attempt to alter contents of /etc/resolvconf was wrong-headed. OMV5 uses systemd-resolvd not resolvconf. There were therefore no contents in /etc/resolvconf which is why my initial
echo "nameserver 22.214.171.124" >> /etc/resolvconf/resolv.conf.d/tail
failed and my subsequent clueless
echo "nameserver 126.96.36.199" >> /etc/resolvconf/resolv.conf.d
apparently worked but actually accomplished nothing. Can't explain why these steps seemed to fix domain resolution (briefly). Hallucinations?
Tried the fix here: SOLVED Upgrade from OMV 4 to OMV5
.. using both 188.8.131.52 and 184.108.40.206; still no joy.
Sadly, failing again with exact same DNS issue. Can't explain how it could revert; perhaps I rebooted? Not that that explains anything.
Well, I for one am putting this on the back burner. I have another machine running OMV4 that has none of these issues and will use it for this sort of network access (NzbGet, etc.) until I feel ready to start pulling hair out again.
I'm just leaving this dispiriting note so no one is disappointed that the fix shown here doesn't last. For now.