Galaxy S3 won't complete boot after Cyanogenmod reversion
After many failures, I finally figured out what was happening. Cyanogenmod's update utility told me about a new snapshot for 10.2. I was currently on 10.1. When I installed the 10.2 "stable" snapshot, I still had the 10.1 gapps on the phone. Evidently this is what caused the boot to fail. After wiping the caches and doing the "factory reset", installing the 10.2 gapps and the 10.2 CGM snapshot, I was able to get the phone to boot successfully.

I don't know why the 10.1.3 reversion failed when I still had 10.1 gapps on it.

I'll update this answer later if battery life is still an issue, but I believe I should be able to revert to 10.1/10.1 if battery still doesn't behave well in the 10.2 snapshot.

While trying the paths, I also tried the CGM 11 (4.4) beta build. Looks pretty sweet, but I couldn't get wifi to work on it, so I went back to CGM 10.2 snapshot.