2) environments where "your survival depends on efficiently communicating and organising" tend to ignore habeus corpus and standards of evidence. Realistically, your survival depends on external factors (your side winning) rather than hoping you and everyone you've been communicating with can withstand torture indefinitely.
2b) digitally signing a message reduces plausible deniability
Ofcourse it'd be great if twitter did the job for you, but I don't necessarily see why it ought to be Twitters position to fulfill this edge case rather than say a protester who knows they are about to go into a life threatening scenario and need to organize--both beforehand and during the escapade.