Start with the ISBoxer 7day trial.

You can set up all you accounts under the same battle net account using the same email. This will allow quick and easy transfers of characters between them, mounts, pets and achievements will be shared across all accounts, you can trade heirloom gear between accounts, etc. This will also allow you to use 1 authenticator for all your accounts.

Yes, you can and should use RAF to get yourself the XP bonuses and such.

If you're using PvE servers, there's probably not that many "bad" servers. I'd probably avoid any that are listed as "recommended" in the Blizzard list, those will generally be low-pop servers, which means a pretty poor economy. You can also check the Guild Forums to see if there's any multiboxing guilds out there you might want to join.