Theoretically, what happens if a flight diverts to a foreign country, needs everyone to leave the plane, and a person is found inadmissible to the country (even if they had a visa)?
For example, what would happen if a person from Iran was on the flight to Vancouver that diverted to Seattle? From what I see in news reports, the passengers did go through immigration.
Depends a lot on the specific situation, but many airport immigration have some sort of "emergency visa" and "quarantine" procedure for this.
Example: I was booked on a flight out of China that got cancelled late. We were still in the airport but we had passed exit immigration. Many passengers were on a single entry visa or a on the second trip for a dual entry visa. These visa had already been invalidated by exit immigration and so these passengers could not legally enter China again. Immigration set up a booth, where these passengers were issued a 24-hour special purpose visa and they got bused to a hotel. I'm not entirely sure but I think they weren't allowed to leave the hotel until they got bused back to the airport.
