I would guess that it will help (eventually) with email deliverability: phishing emails claiming to be from Facebook wouldn't have valid signatures. Yes, DKIM and other stuff already partly does this, but signed emails are much harder to forge!
Yes, but they could just start signing the mail. There's no reason to encrypt the content if proof of origin is what they want to provide.
Signed (and unencrypted) mails are vulnerable to known-plaintext attacks; in other words, it is possible (although not always feasible) to alter their content yet retain a valid signature.