To me "I don't know" is pretty close to "didn't happen" anyway. I mean we will even say "hmm, I don't know about that" to indicate skepticism.
I suppose you are picturing someone who is obstinately refusing to believe or even investigate a claim, but when I hear "null hypothesis" it's really just "you haven't given me any reason to believe this." And that's not quite the same as "I don't know" but it's not that far off either, is it? Maybe it's just me.
Uh, sure they can: he saw an opportunity where he could make a difference and bring a program to light where the NSA was otherwise blatantly lying to Congress and the American people, and he took it.
There is nothing he can or could do to stop the invasion of Ukraine.
Which is to say, he didn't merely oppose US crimes. He brought them to light. Everyone already knows about Ukraine.