Hello. Thanks much for posting such a clear report. I have a similar problem -- but for me it is more rare... like once in a 1-2 week period I have this problem, namely "mouse click stops working totally on both magic trackpad and built-in trackpad, and I have to reset."
-- You post as a reply to your original post that you have solved it because you found that you had another wireless mouse on nearby. You said it was embarrassing that that was the answer. But that doesn't really make sense. just because it is connecting to another wireless mouse doesn't mean that the other mice should stop functioning. It's not really an answer...
-- In any case, it doesn't answer my more intermittent problem. HOWEVER, I will admit that I had an additional bluetooth mouse set up under Bluetooth... but that additional mouse was turned OFF. BUT, just in case it helped, I have now removed the configurations for two bluetooth devices I had under bluetooth (that extra mouse and my mobile phone, which I never connect to anyway.) Perhaps that will decrease the computer getting into that frozen "can't click" state. we will see...
So -- thanks again, your post gave me an idea, namely to clean up my bluetooth configs so that I have only two items, the ones i really use: apple bluetooth keyboard and apple bluetooth magic trackpad. Fingers crossed.