Earlier quoted context omitted.
I find your numbered options fairly absurd. The US has let Snowden linger in the Moscow airport, likely hoping that he'll do something stupid due to the stress. By reminding everyone of the dead man switch, Snowden (via Greenwald) reminds the US that both have a shared interest in Snowden's asylum bid moving along smoothly. Your conjecture above depends entirely on the idea that Snowden is unsafe at the Moscow airpor…
What does it matter where he is? The same numbered points would apply if he was holed up in the back of a Chili's in Des Moines.
2) wildly speculative and wildly inconsistent with all of Greenwald's other behavior.
3) I imagine you have some kind of conspiracy theory to justify this one?
In this case, the simplest answer is that a dead man's switch gives Snowden a bit of additional leverage, and that Greenwald mentioned it to help the reporter interviewing him understand the dynamics of the situation.