Had shut the machine down. It had been trying services for an hour (was the warning?, iirc).
I restarted. Windows offered me the option to fix disks (partition, one at a time), and actually allowed me to safely remove the external drives and replug in so I could fix each partition. I did not get the multivolume zip error I'd also started getting (but, maybe because I've not tried that thumbdrive again yet), and Drivepool started right up, no errors. So I can't get you screenshots of what was going on (but, something working is far better).
Cranked thru the user guide (is there a pdf version I can download, and put on the airgapped / non-networked machine?) - which explains some of the things that weren't readily transparent in the UI.
Additionally, are there some use-cases I can go read? I think I'm going to need to use some deduplication tools, as the only thing that seems to be integrated when copying files to a drivepool are some file/folder validity checks (and there doesn't appear to be an error log, so figuring out where this one invalid file was located looks like a hair-tearing experience).
To easily make a backup, I should add a drive, turn up file duplication (say x2 to x3), after balancing, I can remove the drive - (the files will still be in the hidden drivepool folder?), and it can chill out offline somewhere? Or perhaps not, as the removal options don't clearly state that (they want to migrate files off?)
If files get (inadvertently) removed from the pool, when I re-add the chilled-out drive, will they re-populate to the live drives?