a comedy of errors
And what a comedy it was, indeed! I’m still laughing 
In the end, it turned out to be a permissions problem. As you’ll note here, I’ve been laboring under the requirement to sync to an NTFS partition. Well, the painful lesson in all of this is that Syncthing and Linux and NTFS don’t play well together. Syncthing was choking because it couldn’t run a proper chown where it needed to.
Sure, one might be able to take a bunch of trips 'round Robin Hood’s barn and MAYBE get it to work under NTFS, as @gadget describes in his excellent answer, but why go to the trouble when there’s an easier way (at least for my scenario)?
Bear with me…
Once I had an ext4 partition, I was able to set the directory’s owner to the user under which Syncthing is running, and then to set its permissions to allow read/write by anyone. Once that was done, I created a Syncthing folder and watched in absolute adoration as the synchronized file count started climbing immediately. Lots of nice network traffic, too.
And now I remember how it managed to work under RPi/NTFS all those many years ago (see the other thread). I was running the service under root! For shame, for shame…
So I take back all those nasty things I said about Syncthing’s heel. Apparently somebody came along after Thetis half-finished the job and turned Syncthing upright and doused his foot as well. I must have been looking the other way and missed that.
As far as disaster recovery into a Windows environment, there’s more than one way to skin a cat.
VirtualBox is a Type 2 hypervisor, which means it supports USB pass-through. In the unlikely event of a data disaster at this scale (it’d have to be a pretty serious situation to bring me to the far reaches of my plan like this), I’ll spin up an Ubuntu VM and use it to relay the files to an NTFS partition somewhere.
Either that or I’ll just turn the whole thing around and recover the files with Syncthing, simply the reverse of how they got on the drive in the first place.
Easy, peasy, Japanesey.
Thanks, everybody, for chiming in.