I thought it was all working OK, but I just shut down my PC and booted from my Ventoy USB drive.

Apparently, Ventoy works the way that you suspected, fsmithred. In Ventoy, I first chose "snapshot_20210311_1253.iso", to select my iso snapshot.

That caused a second "LIVE" menu to appear, in which the first (default) of several entries automatically starting before I could read all of the other choices.

Several (~30?) boot-up lines scrolled down the screen, quickly, but eventually began to appear several seconds apart.

The last few lines that appeared said:

Begin: running /scripts/live-pre(something I don''t remember)... done
random: fast-init done
random: crng init done
random: 4 urandom warning(s) missed due to rate limiting

Underneath that, a flashing underscore prompt appeared, and nothing further happened.

After waiting for 2 minutes, I manually shut my PC's power off, removed the Ventoy USB drive, and successfully booted back into my hard drive's installed Beowulf system.

Maybe instead of using Ventoy to start it, my RefractaSnapshot it would work as expected if, as was hinted, I simply copy that iso file directly to an empty, formatted USB drive?

I hesitate to declare victory, because I've never seen or used refractasnapshot-gui before today, but, as far as I can fell, I was successful. I copied the .iso that it created -- which was in /home/snapshot/ to my Ventoy USB drive, then shut down. I started with the Ventoy drive in place, and it gave me 2 menu choices:

1. Beowulf 3.1.0 server (which I had used when I originally installed Beowulf)
2. Devuan Beowulf (the RefractaSnapshot that I had just created of my running system)

When I chose the second menu choice, I was shown several different installation choices (I don't remember what they were. I quit without choosing any of them.

Does that sound like what's supposed to happen -- no LIVE image, but the option to RE-INSTALL my current system, exactly the way that it was installed and configured when I used RefractaSnapshot to create that .iso?

If so, that was so fast and simple that even a guy like me, with a brain the size of a golfball, could do it! 

<update>Many, many thanks to all of you, especially to dice, for your very kind offer of help, and fsmithred (If you are, indeed the person who created the fantastic RefractaSnapshot-gui software that I used).</update>

Thank you very much! I don't think (after suffering 2 strokes) that I still have the concentration/personal memory, etc. that it would take for me to use anything other than some kind of dumb, point-and-click software (if that kind of thing even exists), but I will go through the entire thread to which you linked me, and see if I find anything that I think I could do. Thanks, again -- to me, that thread contains valuable information! 

THANK YOU! Using your excellent, valuable, and previously un-findable (to me) advice, I was able to easily get my system back to using the "xfce" folder for desktop background/wallpaper images! Yay! 

Thank you! That makes sense! 

Like I'm guessing many (most?) other users, I use the standard repos, as suggested by the Devuan site.

In your Google search, put the word "Devuan" in quote marks.