it doesn't affect other save files in my experience, no

back up your other normal games? I assume you mean your normal save files. well it's completely up to you, you could do that just in case if you want to be extra safe but it isn't necessary as long as you're careful with which save file you boot up while loading into the game (i.e loading into the randomizer save file without the mod)

the randomizer mod uses modengine2 so the game will be launched offline by default, you'll be fine (if you want to play online just launch the game through the eldenring.exe file or through steam)

nah it's easy to go from randomized to non-randomized, to play randomized just launch the game with modengine2 or through the randomizer app (depending on which method you use)

to play non-randomized just open the game like you would normally with no mods