That having been said, artillery pounding densely packed cities will cause terrible casualties. A city with skyscrapers has never been shelled that heavily before, you'd see the whole skyline collapse in short order. Even so, the shells alone would only, like you said, probably kill off one or two hundred thousand before they could get out of town.
Many casualties will come from chemical weapons. If you have any notion that North Korea won't use them for some reason, part with it immediately. Their doctrine calls for persistant strikes in population centers south of Seoul, and non-persistant strikes on troop and civil concentrations in the path of their advance.
A massive refugee problem will soon erupt, and disease and starvation will claim many as food distribution networks on the densly populated peninsula break down. This is what generally kills off most civilians in war, anyways. If the war drags on for any amount of time, I'd expect to see civilian casualties (SK, that is) number around one million, if we could wrap it up in a few weeks, then it would be less.
You make several excellent points here. All that I would add to your argument is the mayhem which would be caused by PDRK infiltrators in the early hours of an attack. North Korea places an inordinate emphasis on its very large special forces, and has demonstrated the absolute ease with which its agents can infiltrate across (beneath) the DMZ. Seoul would be a very miserable place in the event of war.