To answer the 2nd question first, some USB drives will occasionally allow boot on some Macs. But in general they don't work.
Special FileVault handling is something we'll consider for the future -- but, no, root can't just mount it. It's encrypted, and root can't get by the encryption.
You might find it better to use something like
Knox. FileVault is kind of massive overkill for most people who want to encrypt a few things -- I mean, why encrypt your MP3s?
With Knox, you can encrypt those few things, but leave most of your stuff "normal", which will improve your backup speeds and also improve safety, since encrypted images are inherently less stable than going right to the drive...