WebPG or Cr-gpg perhaps can handle the GPG part probably.

I'm not so sure about the not saving Drafts, that would require rewriting some bits of Javascript which might break other stuff.

As for key exchange, that shouldn't be handle by an extension. If you want to encrypt your e-mail so the other party can read it, encrypt it with their public key, which by definition is public and can be uploaded to a keyserver.