Also, we don't know what the data contains. So even if he does give out the key and the media start publishing, who's to say what comes out wont be in the public interest?
I think it's a bit early to say this strategy has massively hurt him.
Snowden is coming to light as a complex character, not some altruistic idealist who expects a pardon and a ticker-tape parade. Phrases that come to mind are "David vs. Goliath," "Catch me if you Can," and "John Le Carre." I love it because it's a more interesting story this way.
This isn't his ace-in-the-hole insurance for him as much as it is insurance that killing him can't stop the release of the information. That makes him far less valuable as a target for "extralegal field maneuvers".
By distributing the file, he's announced that the information has become a hydra - cutting off its head won't kill it. This won't stop the US from pursuing him, but it guarantees that his goals will be met regardless of his personal safety.