Disk Utility Usage

My late 2012 iMac with Fusion Drive was running Sierra and working fine, no problems. I upgraded to HS (V10.13.1) and have some odd problems. I use FileVault 2.

I have been running Disk Utility to try to verify the HD (no reason to think an OS upgrade would damage the HD, but just in case ....)

1) When I booted into the recovery partition DU First Aid won't work, on either base drive or SSD. Why not?

2) I booted to a thumb drive with the installer app, and tried DU First Aid. Same exact result, DU won't work on either base drive or SSD.

3) DU First Aid works just fine from inside HS. I am wondering if the results are reliable inside HS? In particular, every time I run it the number of "Load and Verify Transaction Segment" changes and the report of "Incorporate xx newer non-checkpoint transactions" changes the reported 'xx' value. What is going on there? The log indicates no problems found.

I also tried booting to safe mode a couple of times, as that is alleged to try to fix certain HD problems. But there was no log I could find.

Lastly, a couple of odd problems still persist. They are minor. Just trying to understand DU First Aid. And the Apple articles I can find don't address my questions.