Good questions. I have a theory that when Q was human, maybe he picked up some sort of weakness and it has eventually spread to the Continuum.I wonder if this Q is the first to be dying or its affecting others in the continuum as well?
Q no longer has his powers as we saw at the end of 2.4, but he had them in 2.1/2.2 when moving Picard and crew to the altered timeline and appearing, disappearing. So did he have powers/change the timeline, used his powers to save Picard in 2.1 and he had them in 2.2, then he went back to the past to stop Picard from fixing the timeline..and then lost his powers? Just curious about the timeline of Q here.
At the end of 2.4, Q was genuinely surprised his powers didn't work. So somewhere between saving Picard & co. and them going back in time, something really got borked. Perhaps Q had a sense of it when he told Picard, "I am but a suture in the wound"?
Remember that Quinn in Voyager's 'Death Wish' told Tuvok the Q aren't omnipotent no matter what they say. Q de Lancie has also made the occasional statement in episodes which suggests he doesn't know everything. Or that he can't always fix everything. I feel that the Q are best able to do what they do when they're controlling the game as it were. Like someone else said above, they live in their own timeline of sorts.