If North Korea disintegrates, both China and South Korea be ready to handle millions of cannibal zombies. China will just mow down, but the democratic South Korea might be flooded with their own refugee problem... This time, an atheist wave of starving masses. Equally vindictive and resentful of their opponents as ISIS refugees are.
Doubtful. The Koreans that have defected from the DPRK to the ROK since 1953 haven’t acted that way.
The DMZ minefields would work in SK's favor, in that case. China would have the bigger problem on its hands.