So at what point does the onus shift to the airline? What if you arrive 6 hours before the cutoff, but staffing levels don't allow them to get to you in time? Clearly there's a point at which it's no longer the passenger's fault.

Either way, if you arrive at the check-in counter even at T-65, and you have the required documents, I don't see how that's the passenger's fault. It seems there's a dispute over when the passenger arrived, but even if we accept AC's version, the aunt arrived in time.

I've dealt with enough ignorance from check-in agents over "visa issues" ("Sir, you need a green card to transit the US") that even I get worried when I'm completing check-in in any country outside Canada and the US, so I can appreciate how frustrating this is.