They’ll have to kill the majority of the population. Right now the mindset in the south is they are one country divided. If NK attacks the attitude will change rapidly.
NKs plan is to destroy SK with artillery and chemical weapons. The nukes are reserved for the US.
Hopefully its not going to be proven one way or another but I’m not convinced the South Koreans epically those less than 40 or so would not simply roll over after a strike on Seoul. It would be much easier for the North to pull off than a long range attack on the US...