If you want to see what irresponsible journalism is like, read Benji Smith's book "Abandoned Ship", especially the part about "Jenna McJournalist". Laura hasn't (to our knowledge) behaved like that.

There's probably enough trust between them now that he isn't going to get upset if a hash of his email doesn't match a hash of her quote of him. She's also likely smart enough to realize that he could drop her at any time and the other news organizations would be lined to up for exclusive access. So she's going to act responsibly towards her source.

I don't think she would run the risk of ruining her credibility like that.

Snowden can say something if he's being misquoted. This is just FUD.

I'd assume that in addition to encryption, Snowden signed the email, so no MITM attacks.