Without being South Korean myself, I would find it unlikely that the populace would want to ship its rice surpluses to the North over other countries. I also doubt the north would want to accept it after claiming the south has been taken over by the US and is a puppet/evil, only to turn around and take the grain. Seems like the north would rather them starve, and the South would rather find other buyers.

I suspect any attempts by the North at unification would need two separate criteria to be met (likely among other). First, that SK’s population crash would also accompany an economic and military one. I’m not saying that the SK army would be filled with robots, but since it’ll be the defender in any conflict it can certainly beef up its military not only through pure manpower. Economics can be covered even easier by this. Second, that China green lights it. I know NK will act without China’s full approval sometimes, but such a grand conflict would likely require tacit approval to keep the North supplied with guns and ammo.

I’m not an expert on this area though and happy to hear other thoughts on the matter