Overwriting your emunand with an older one will get you exactly what you had when you made it, including games, save games and settings.
Your save game data is stored in the emunand so if you overwrite the current one with an older one, you will loose that data.
Afaik, the CFW ignores the fuse count of the Switch so you should be able to boot in an older firmware version.
Only the cart reader has a downgrade protection so you can't use the cartreader for FW9 again if you were on FW20.