Thanks for your thoughts; they're so much more coherent than mine, they almost make troubleshooting fun.
EtreCheck tells me this about 3rd party kernel extensions:
Kernel Extensions: ⓘ
/System/Library/Extensions
[loaded] com.protech.NoSleep (1.3.0 - SDK 10.7) [Lookup]
[loaded] org.dungeon.driver.SATSMARTDriver (0.8.1 - SDK 10.6) [Lookup]
No Sleep predates the problem by close to 5 years (and under any circumstances, it's more important than correcing the problem), and the problem predates SATSMARTDriver by close to 2 years, so I didn't experiment with either.
But, on a lark, I repaired home folder permissions and then reset NVRAM again, and the problem appears to have been solved: created and modified dates appear in Get Info panes in both my boot volume and my test user account, and that has survived a restart.
