How tight is this connection? If you mis the YHZ-YYZ flight and get put on a later one, will you still make YYZ-IAH? If not, you should note that the return IAH-YYZ flight will be cancelled as you will be deemed no-show for the first segment.

In terms of your first question, you need to check whether WestJet and AC interline. If they do, you should be able to check your luggage all the way to IAH. If they don't you'll have to pick it up in YYZ and check in or bag drop for the IAH flight. If you don't pick it up personally and the bag is only tagged to YYZ, it'll stay there and not make its way to IAH.