Yes you would have to clear customs in Canada before flying out again. Unfortunately the North American aiports have never adapted to the system many other world airports use. This may be simply because it is not as common to fly in and out of Canada and the US as it is in London or Amersterdam.

Late November is unlikely to have weather delays in Atlanta but you might have in Toronto.

My experience on overseas flights is that Air Canada has better service than Delta. But are you sure it is Air Canada and not a code share with Alitalia? My wife flew Toronto - Milan a couple months ago and the flight was actually Alitalia. She said it was very good though. (I have never flown this airline.)

As for an awful airport in Toronto, I wouldn't say it is anymore difficult than Atlanta. I have flown many times from both. They are both easily manageable and well signed. This wouldn't really be a concern of mine.

Hope some of this helps.

Jude