1) Not normally; as I understand it, the firmware flash will in most cases overwrite the memory. I would expect that the settings are on the same ROM chip.
2) Restoring a saved config file may or may not work, depending on whether it requires the same firmware version or whether the new version is substantially different. Unknown territory.
3) A very good idea, I would say. Covers your back in case any of the above issues crop up, or that the new revision is not 100% solid. The trouble is that most router manufacturers do not offer anything beyond the latest version. I cover that by downloading a copy of the original for any new router at the time of installation, but that's academic here of course.
I think you should just save the config file anyway, but write down your settings and option choices in case they are wiped and you need to re-enter them. I would expect the new firmware rev to be okay.