If NK pulls off a surprise attack, the South is fairly screwed even if we shot down every single missile aimed at them. We can’t shoot down artillery shells, and the Kims have enough artillery stashed under mountain bunkers within range of Seoul to pretty much level the place. Ships can’t do much when the enemy is dug in like a 20 minute drive from your capital.
I think a lot of NK capability is overrated. In my scenario they may even invade as far as the Han River but that will be tough for them to hold in the longer run.