(5) Greenwald is thinking two moves ahead. Now that US adversaries have a motive to kill his source, the US finally has a motive to protect his source.
Russia is not going to let harm come to Snowden in their airport or on their airplane. No one's going to stab him with a poisoned umbrella or anything.
Greenwald and co. have been consistent since the beginning that precautions had been taken with the trove of documents to ensure the reporting would not be suppressed were something to happen to Snowden. It's recently been sensationalized as a "dead man's switch", but it's not new info.
So I think you're right.
Look at how reflexive the US has been with all the diplomatic threats and goofups compared to Snowden's team's patient and careful tactics. The US hasn't been on the receiving end of effective hardball tactics in a long long time. It's taking a while to sink in that they need a better strategy than the standard prosecutorial bullying.
Has he laid out in such stark terms, though? He has been saying the documents could cause "grievous damage" or something along those lines, but that's a far cry from "more damage to the US government in a minute alone than anyone else has ever had in the history of the United States."