Practically speaking, the best response to a sudden collapse of Nork would be massive food aid, ships full, to its various ports. This would come with the simple message: “Stay here for the time being, because this is where the food is. Eat as much as you can.”
The next step is to create gigantic “food camps”, preparing the food for them, as they have minimal fuel to cook with, encouraging people to eat big meals there, then transport a lot of cooked food home for themselves and anyone else that cannot travel. As more and more food arrives, build more such places inland, to reduce travel time.
Relatively speaking, food buys a lot of time to get things moving again, so tens of thousands of SKor social service people would be deployed all over Nork to facilitate the creation of town governments, communications, supply routes, medical aid, identity papers to figure out who is where and how much of everything is needed hither and yon.
The better the initial response, the more manageable the refugees. With even a minimum of order reestablished, there would also be a huge outflow of illegal alien Koreans from China back to Nork, which also has to be considered.
Oddly enough, food is very inexpensive, and both the US and Europe have perpetual enormous overstocks they could dump quickly, and would even save money, because warehousing it until it rots is expensive.
Excellent post.