The pre-clearance bit is a convenient thing that I'm genuinely appreciative of - thanks to our governments and airlines for figuring out how to make that work, thus enabling lots of secondary airports to receive Canadian flights when they otherwise wouldn't have them. But no question about it - you're still in Canada when you're in the little preclearance corrals in Canadian airports. Every now and then I'll see a reference to Canada either giving up sovereign territory or the U.S. taking it in the form of these little corrals, and that isn't the case. They aren't like little embassies or anything.

It seems like there's little question that if you're on the ground, in an airport, you're in that country. If U.S. officials want to apprehend you, they can do it airside or landside - they don't have to wait for you to stand in the immigration queue (or choose to hide out in the toilets).

I don't know about airspace though. If you're on a flight from YYZ to MEX, are you ever in the United States? If U.S. intelligence happens to know that someone wanted for murder in the U.S. is on that flight, can they order it to land in Ohio and board the plane to make the arrest? (Assume it's an AC or Aeromexico jet...)