… I wonder if it would be worth your while to boot into your Recovery partition and repair home permissions (if the functionality still exists and is still located there [I haven't looked in ages.])?
Your suggestion was a good one, even though it didn’t work. You couldn’t know that because I hadn’t included my system specs: iMac Retina late 2014, 4GHz i7, 16GB RAM, Mac OS X 10.12.6. Sierra's version of DU doesn’t have the option to repair home permissions directly from the RP, but it forced me to find a way to do it regardless.
The solution is described in
Something odd you can’t fix? Sierra re-introduces repairing permissions. The Finder’s
Get Info_enclosed items repair took about 40 minutes, the Terminal follow-up somewhere between 5 and 10. No error -69841, and the errant files are now readable by Preview. I still have no idea exactly why GraphicConverter worked on the florfed files, but I have my suspicions... It's my go-to app for problem image files, hence my bringing it up here.
