Makes me wonder if we'll see another "Operation Yellow Ribbon".

https://en.wikipedia.org/wiki/Operation_Yellow_Ribbon

Alas, the Canadian goodwill level may not be as high this time.

Doesn't seem like it; I've seen several international flights land in JFK alone since this started. (On the other hand, these might simply not have enough fuel left to divert to any non-US airport.)

Not in the industry, but I think ground stops might actually not be a regular/automated procedural thing for inbound international flights in the way that they are for domestic flights.

As for asking already airborne flights to turn around, those coming in over the ocean might simply not have a non-US alternate airport available with their fuel remaining, so that's often not an option.

Every flight over an ocean has a set of pre-planned diversion airports.

Yes, but most of them are not viable in the late stages of a flight. Planes aren't carrying enough fuel to cross an ocean for a second time; most don't even have the capacity.

Please suggest an appropriate international alternate for a plane just about to land in Denver with an hour of fuel remaining.

Alternate airports are picked based with regards to all kinds of contingencies (many of them meteorological), but "the US closes literally every airport" is not one of them.

That likely falls within the case-by-case basis the FAA mentioned, and would likely be allowed to land at a U.S. airport that has capacity.

I'm not aware of a requirement that you need to always be able to divert to a different country.