Okay, we have some good news, some bad news, and a "Well, duh" moment.
Good news - the issue is resolved and the offending document has been banished along with a bunch of others. Along the way I also learned that the items could not be removed by booting from a different volume.
Bad news - I appear to have burned up a lot of peoples' time for naught.
"Well, duh" moment - I booted from my backup, made invisibles visible, and tried unsuccessfully to remove the documents. Thinking they may be locked I used 'Get Info' to unlock. Then I noticed that 'Get Info' included this pertinent datum: "/Volumes/Time Machine/Backups.backupdb/Dad’s iMac/".
The offending documents weren't even on my main drive.
Anyway, long story short, I erased my Time Machine drive, recorded it anew, and all the bad stuff is now gone.
Well, duh.
