bubbers44:
KSNA and KDCA are very similar in how they operate. I was based at KSNA with an airline. All departures stopped at 10:00PM and all arrivals stopped at 11:00 PM for jets. That was because of noise abatement policies. The tower was then closed and it became an uncontrolled airport. I have landed after 11:00 many times in twin props with no tower. Once in a Citation but that's another story. We were very quiet. Going from controlled to uncontrolled was a non event.
Those two airports are similar only as to having curfews. As you say, KSNA closes and the Class C airspace disappears. KDCA, OTOH, is a full-time Class B airport.
As to taking off and landing at uncontrolled airports some of us who flew a lot of light aircraft time in addition to the airline, did a lot of uncontrolled airport flying.
In all my years of flight aircraft and airline flying, and instrument flight and ground instruction, I never heard of a airport with a control tower that was published as operating suddenly becoming an uncontrolled airport because pilots couldn't contact the tower.
Before the incident at issue is sorted out, I suspect that Potomac TRACON will be deemed to have operated outside its authority. And, because of the manner in which the TRACON communicated with the two affected flights, that alone will probably prevent any violations being filed against the flight crews.