1. eBoot boots up in a special recovery mode, rebuilds SBFS, inserts all the files into it, and re-deploys all the user accounts into it, totally rebuilding the boot environment. Restore SBR just replaces sector 0 of the boot drive with the stored boot record
2. No user side files are touched in either case.
1) I would say emergency boot will point to backup sbfs (contain key) which bypass user authentication then after os boot's it's will rebuilt the SBFS (which contain user & key) by conecting to MEE Server.
2) no files replaced but i believe code or pointing in SBR/MBR located in sector 0