It would cause mass casualties but by no means would the metropolis be 'obliterated'. There certainly wouldn't be 20 million dead unless everyone within the Greater Seoul area rushed north and threw themselves into the beaten-zone.
Assuming, of course, that the artillery troops follow their orders to fire.
Even assuming that there isn't a concentrated effort to destroy these emplacements quickly, it seems the amount of damage they could cause would be fairly limited by operational constraints[1]
[1] http://nationalinterest.org/blog/the-buzz/could-north-korea-...
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.
The only real issue we would have to worry about is NK using a nuke on Seoul or Tokyo. That is the major issue and it is why NK will never give up on their weapons. It is the only effect threat they have.
The US planners have sorted out the conventional war issues a long time ago. It is figuring out how to be 100% sure to remove the Nukes from the equation that has stopped us from just stepping on Kim like a roach and he knows that. That is the reason for the Nukes.
Let's hope it doesn't come to that.
