As soon as they open fire, the counter-battery game begins. You can be sure the allies have photographed and analyzed every strip of dirt along the DMZ, looking for prepared positions so they can anticipate where the guns are and where they'd move to after a salvo. They'd be tracking the rounds as they come in and plotting where it originated from. 72 hours is a good estimate for how long that artillery battle would last; same estimate I've seen elsewhere.
But the concern of casualties are not necessarily from shells impacting, but the panic and disruption in order that comes from it. NK can cause enormous hardship just by flinging a few shells. They can hide many of their batteries and string out the battle by revealing pieces on day 3, 4, etc,. Even if they get destroyed, and even if they only get a few shells off, you'll have a news report saying shells are still falling on Seoul, and it will be a tremendous morale hit.
But the window is closing on how well the NK artillery will continue to be a credible threat, hence the rush for nuclear weapons.
