Greetings!
I found on kayak a good one way deal from IAH - YYZ with a long tedious connection in RSW. Typically I wouldn’t take it, but this time I had no choice. RSW - YYZ is operation by AC - it’s a 016 ticket.
For the record, Kayak sent me to the UA page where I completed my booking.
I have two questions:

1. on UA website when I go to “change flight”, it only gives me UA/AC options, and never UA direct flights. Why is that?
2. 2 hours after I land in RSW, There’s an almost a completely empty AC flight, earlier than my original one. I have only a carry on. Within the AC airline policy, can I stand by and get on the early flight without paying any fare difference?
3. After checkin, will I be able to do a SDC to a direct IAH-YYZ flight if there’s room?

The flight is this Sunday

Thank you in advance!
R