Well, turns out there's ONE MORE WRINKLE in the DRM rules, that I did not know.
The knowledgeable group over on The Green Button have provided the explanation:
"The DRM keys change over time. Your image was too old and you ended up resetting your DRM keys to last month, so more recent recordings don't play anymore as they were recorded with keys that you wiped from your system. If you ever want to restore from a backup and not lose the ability to watch recent recordings, you MUST make a backup of your OS partition every night."
In fact, that is EXACTLY what happened. The one mysterious copy-protected WTV recording that I COULD play after the system image restore was dated 2/5. The system image itself was dated 2/9. So anything recorded 2/9 or before WOULD be playable on the newly restored system.
But anything recorded after 2/9 (such as the other 5 files, which were from the last week or two) had a time-based DRM key which did not work with the 2/9 restored system.
Hence my results.
And, apparently, if you want to retain the ability to always be able to play EVERYTHING in your recordings folder, you absolutely must take a system image backup EVERY NIGHT!!! That's the only way to ensure that all your WTV files are OLDER THAN THE SYSTEM IMAGE, and hence the DRM keys will still work.
New information that I didn't know before.
(fortunately, I haven't really lost any of those 6 programs)