I'm not going to lie. This could be a bit of a challenge. I'm not sure there's much you can do now, but definitely ask about what service the airline can provide to help you with your transfer in Toronto. Given your MIL's ankle and hip, they should provide you with handicapped golf-cart shuttle service to help you navigate the terminal(s) without her walking speed being an issue. They may offer additional service to help you transfer your bags to the US TAS / customs and immigration screening area. I'm not sure there is any value in splitting up your party, as you need to have all your bags together and enter the US customs and security zone as a group.
Toronto is a sprawling airport, with multiple terminals. However, since they demolished Terminal 2 and modernized Terminals 1 and 3, I think passengers flow through more efficiently. However, it's still highly likely that you will have to transfer between terminal (check your flight information -- it should indicate indicate the terminals for arrival and departure flights. However, there is now a rail link system connecting the terminals, likely similar to what is at Orlando International Airport.
Will they wait for you if there's an issue? Perhaps. But in my own experience, they don't. Toronto is a very busy airport and they like to keep things on schedule.
My only experience with this in Toronto was coming back from Orlando, going through Canadian customs, and catching a connecting flight to another Canadian city in 2005. We were exhausted, and the airport seemed huge and unorganized. We had to take a bus between terminals, and I can't remember exactly where we had to pick up and drop off our luggage, but it seemed like a hike. It's likely better now, but I've avoided these types of connections ever since.
You are probably right to trust Air Canada, but if you want to ask more detailed questions about connecting in Toronto, you should make a new post on either the
Canadian Trip Planning & Community Board or the
Transportation Board. The Canadian board might get you better results because many of the bloggers on that board are very familiar with the Toronto airport.