Following the advice given via PM from @bcopeland, I visited the following URL for my hub:
/hub/advanced/installZipgateway718
...waited until I got the message "success, please reboot" (your browser will just sit there loading until you get a result), and then rebooted. After that, everything was working again. It seems the software for the z-wave chip got corrupted, perhaps after an incomplete or failed firmware update.
After I rebooted, for good measure I also shut down the hub, removed power for 30 seconds, and powered it back up, just to be sure the z-wave radio was fully reset. That was mostly because I had previously futzed with the country/frequency setting for z-wave (back when I was flailing, trying anything to fix it), so I thought it best to ensure the z-wave radio was properly powered-off after I set it back to the correct frequency setting.
Hopefully this will help others, who get the dreaded "Z-Wave Unresponsive" message and also find the "/hub/zwaveVersion" page returns a 404 error.
Thanks for your help, @bcopeland, @csteele, and @jtp10181 !
