Consider the following scenarios:

Flying YYZ-CVG-RDU. Pax has checked in, but arrives at the gate just a few minutes after the boarding door has closed. There are other, later connections. Will DL SDC to another routing if there are seats? (The outbound is at 5:20PM, the latest departures are around 7PM, so there is no issue of a 3-hour window.)

(Issue here is that my wife gets off work at 3:05 and can't leave early, 2 hours should be enough for her to make it to the gate even with security and passport control, but sometimes s*** happens.)

Same routing, YYZ-CVG is delayed and probably won't make a tight connection in CVG. I assume they will reroute on DL if it is wx, if there is a seat available that night, or if it is mx they will probably reroute on another carrier?

Missing the plane is more flat tire rule than necessarily SDC, so at DL's discretion they could let your non status (?) wife fly standby. Even for MX, they will try to rebook on DL or SkyTeam rather than other carriers, which they are likely to avoid unless pushed, except maybe for a HVC or expensive ticket (which the passenger really could change on their own as they wish).

They may charge the $50 SDC fee if she is not a GM or higher, assuming there are seats on the later flight. If she is running late and thinks she is going to miss it, then I'd recommend calling before missing the flight. If it ends up full or oversold, then she could be stuck there.

If she misconnects because of WX or MX they'll reroute or put her on the next available flight. Unlikely to be rerouted on another carrier unless MX and she has status, and there are no other options.

Yes, wife is non-status. Well, pretty high status in my eyes, but not in DL's.

Mx is a rarity anyway, and often when it is really mx they try to call it wx anyway.

Just wanted to be sure of her options, to make sure she isn't getting stuck in YYZ.

Why not just book her on the later flight from the beginning and avoid the anxiety of rushing to the airport from work? This would solve the problem, wouldn't it?

^

Calling isn't really an option, as she doesn't have a cell phone, and stopping to call from a pay phone would take time that could be used to make the flight.

I hadn't considered the oversold question. That could be a problem. The DL site says "For international flights, you must be at the boarding gate at least 45 minutes before your scheduled departure time." In practice these transborder hops are more like domestic flights, but that won't keep them from cancelling her out at 44 minutes.

Can she check-in online ahead of time?

Yeah my concern would be where her original flight is oversold, and people volunteer for the later which fills that one up... leaving no seats to get on the later if she missed the original.

Not sure how often those YYZ-CVG flights run full. Perhaps someone else has experience with that?

She will definitely check in ahead of time. She has a NEXUS card and an Amex Plat which gets her in the express security line. There are later connections through LGA (but leaves just 10 minutes later, first leg is on Westjet), DTW, and ATL.

It just occurred to me, that in case of disaster, I can get her a ticket on Air Canada. I don't know if they would be flexible enough to rebook her. I can get Air Canada tickets at the last minute, the reason she's on DL at all is because the AC schedules are very bad in the winter. But let's say it's Nov. 2 and she can't make it at all. I wonder if they would reschedule her for, say, May 17. And then I get her an AC ticket departing Nov. 2 and returning May 20. They're usually flexible in IRROPS cases, but I'm not sure if they'd rebook for 6 months later.

True. But, if the same fare class (T, U, etc) is available on all segments you can be confirmed under the flat tire rule. If not you can choose to pay the SDC fee as long as they is availability in the cabin you purchased. Looking at the OP..she would have two reroutes after the 5:20 to CVG. The 6:40 to ATL (the advantage here is that both flights are mainline acft and, at this time, showing abt 90 seats each avail) or the 7:20 via DTW. Actually, the DTW conx has the shortest total flt time but the latest arrival time in RDU. Don't think there would be enough time to process the SDC for the LGA flight unless she just decided to have it rebooked at the ticket counter when she arrives. Hopefully...all she has is carry-on(s).

She does only have carry-ons. The LGA flight is only 10 minutes after the CVG, but since the ATL flight is an hour after that, I was going to tell her to try to run don't walk to the LGA gate, just in case. I didn't even know the flat tire rule was still in effect (I know it was never an official rule). It helps to know that the COS determines whether they should charge the SDC fee or not. Despite the fact that she does the flying, I'm the one who's familiar with the rules and regulations.

Actually the whole problem is not the outbound so much as the return. There is an Air Canada nonstop at 9:20 that arrives at 11:15, and she'd much rather be on that than chance a 5:20 flight with a connection. But on Sundays, AC has cancelled their afternoon Sunday flight from November through April, so the latest return RDU-YYZ is at 11:40AM, while DL has a lot of afternoon connections.