Since Q knows about the myriad of alternate realities, is he tormenting the Alternate Picard and TNG crews as well?
Not necessarily. At least in the Dark Mirror timeline Q did not interfere with the Enterprise-D crew. Also, he never appeared in any of the other Mirror Universe stories, now that I come to think of it. At least The Eternal Tide confirms there's only one Q Continuum in the multiverse, so there is only one version of each Q.
But weren't there Trelanes for every reality? And in Q-Squared Trelane is a Q (and in fact... Spoiler: ending Possibly Q's son) ... so surely there should have been at least four versions of Q too?
^Except the idea of Trelane being a "child" Q was roundly de-canonized when "The Q and the Grey" established that the Q had never procreated before. Not to mention that it was inconsistent with "The Squire of Gothos" all along, since Trelane had much more limited abilities than any Q; he needed technological assistance to do his tricks, and seemed nearly powerless without it.
That's beside the point. According to Q-Squared and thus in the context of that story, he is. And surely since there was a Trelane for each timeline, there should be a Q for each too.
No, it's exactly the point. The point is that you can't assume old books are in the modern novel continuity, because the majority of them have been contradicted by one thing or another. The question was about what Q & A, and thus the modern novelverse, suggested/established about Q and the multiverse. Q Squared is not part of that continuity, because it's been contradicted by canon. So it's not relevant here. The real answer is that the producers just forgot or chose to ignore "True Q" (and I can't blame them -- it was a really silly idea). The in-story rationalization is that they had renounced their status as Q and were technically human when they had their child, so it didn't count.
Oh wait, my bad. My brain blipped and I somehow misread the thread title as Q-Squared instead of Q&A.
True Q doesn't count, since Amanda Rogers was born as a biological human and she was conceived in the normal human way. So it took awhile for her Qness to assert itself. As for alternate realities: A counterpart of Q appeared in at least one universe in Q-Squared, so I guess there are individual Q for every universe. Spoiler: Q-Squared The Q of the alternate "Track A" becomes trapped in the Galactic Barrier and is responsible for what happens in "Where No Man Has Gone Before" (it's Q who infects Gary Mitchell and Elizabeth Dehner) .
Remember what Quinn said. We shouldn't think of Q as omnipotent, even if one of them might say they are.
I don't know if this has been contradicted by canon, but IIRC in Greg Cox's Q Continuum trilogy, Q and Picard were observing Q (at an earlier time). So in this trilogy at least, multiple versions of the same Q are possible.