SK threatening to nuke Seoul would make the Kims laugh themselves to sleep. Why not threaten to nuke Pyongyang instead?
The North may have been shielded from the rest of civilization to an unheard of degree, but assuming they are aware of McDonalds on every streetcorner, supermarkets overflowing with fresh food, fish and vegetables, opulence that is common to the south like TV free speech, better housing, automobiles and such and with all that in mind can one logically be lead to believe that the Norks want it destroyed?
A society falling apart from its isolation, eating tree bark, forced conscription, total state control of their lives from birth. But give them a glimpse of whats past their border and I think they would rather defect in mass than be cannon fodder.
By sending out the message that what they crave, what will fulfill their dreams, their ambitions and hope for a free society that will be utterly destroyed will indeed cause disruption in the ranks.