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

1. Probably because you don't have an HOU-YTO fare; you have an HOU-RSW fare plus a RSW-YTO fare. Check the fare rules, which should be available from within your itinerary. Are you in the same fare class on both segments? Are you spending more than 4 hours in Fort Myers?
2. That's a question for the AC forum.
3. Probably not, for the same reason as #1.

As for question 2, ACs policy has changed a bunch of times over the years, and I can’t keep track. Typically, I beleive for US-Canada and vv., standby to an earlier flight is probably based on your fare brand with AC and in most cases, for a basic or standard fare, likely not available on your route (usually for US I think it’s mostly limited to LGA and maybe EWR-YYZ), while on flex and lattitude it’s more likely to be allowed. Check your itinerary on the AC site to see what branded fare you have - it’s likely a standard. YMMV. Never hurts to ask, though they may deny you, especially if you have a checked bag.

as for SDC, if it in indeed a ‘broken’ fare, then SDC to a direct UA flight won’t be an option via the kiosk/app. An agent might grant that request, but that would be at their discretion, and based on fare class availability. YMMV, of course.

You are 100% correct! UA is a K fare, AC is a G fare. That's probably the reason! Thank you for helping me understand!
Would a SDC in those conditions be possible?

You can ask about SDC, but expect the answer to be 'no'. The app definitely won't allow it. Technically, you don't have a contract to travel from Houston to Toronto, so there's some extra work on their end to reconcile it after the fact to make sure they get paid properly. If you have a cancelation or significant schedule change, that might open up the nonstop option.

Very doubtful.

To be fair, a willing UA agent at IAH should be able to do this no problem, presuming availability. Whether one finds a willing agent is a completely different question.

That’s not really true. It’s not even clear what “availability” means in this context; OP is in K on IAH-RSW but G on RSW-YYZ. UA can print an IAH-YYZ boarding pass, but it won’t validate because there’s no HOU-YTO fare to match it.

All of this is possible — and standard in IRROPS — but it’s definitely not “no problem.”

Thanks all.
It’s worth trying.
I figured my best shot is to beg the gate agent in RSW to go on the 12:20 flight. Judging by the available fare classes, it’s a rather empty flight.

the takeaway here is even if you did find a cheaper fare and it does refer you to the airline website, check the fare classes and understand that two fares in one ticket may turn into a prob…

Quick update - while it looked a bit hopeless, begging the CS rep at the premier desk to help me arrive at YYZ earlier, I asked her to check with a supervisor whom eventually approved changing my flight to the direct IAH-YYZ flight.

"There's a crack in everything, that's how light gets in" (L Cohen)...