A domestic flight on 12 November should be ok. The closures or "bandhs" in Nepal are simply a show of strength and tourists are not targeted. Buses and vans often travel without incident during bandhs with a sign saying "Only Tourist".

As has been said before, the only "danger" is inconvenience and delay in travel schedule unless you flaunt the system and take a taxi on a bandh day or try to take pictures of a kerfuffle.

The problem is that these events can occur without advance warning. You can walk out of your hotel in the morning to discover that there is no transport available. If that happens the best solution is to hang out for a day and try tomorrow. Alternatively, there are often government buses that operate from Thamel to the airport with armed police as guards.