Thank you for trying out Westjet! I am sure you will have a great flight! To answer a few of your questions, from the perspective of a WJ flight attendant:
I know WJ has a BOB product, we're hoping that the cheese tray is available so that we can feed her on board
This is only currently available on a couple of flights as we are in the testing phase of this product. It probably won't be available on your flight, I am sorry to say! There is not really anything suitable for toddlers that is healthy on your flight, so I'd recommend bringing your own.
So, we've booked 4 seats on WJ, daughter gets her own seat. We bought the CARES safety harness in lieu of packing her massive Britax Marathon carseat on board...
The CARES safety harness is new for us and many flight attendants may not be familiar with it, so make sure you tell the customer service agent at checkin or the gate to add this to the comments for your booking on the "Specials Manifest" so the flight attendants are aware of it ahead of you boarding.
Does WJ offer early seat-selection, or is this something we can only arrange at the airport? As WJ is out of T3 at YYZ (haven't been there in years), is there a separate WJ check-in in international, or just a general WJ check-in area?
I would phone the reservations line to confirm whether you may or may not pre-seat assign yourself as you are travelling with an infant. I have been told that customer services agents pre-seat assign people with infants, reduced mobility, etc the night before the flight. Best to call in and check first. WJ is out of T3 and there are separate checkins for US departures but I think the domestic and international checkins are beside each other near the domestic security checkpoint.
I'd also check with WJ about the carryon and checked baggage limits for the MBJ route specifically as they have changed from the norm for that route only.
I hope you have a fabulous mini vacation! Feel free to post again if you have any more questions for me! Cheers!