The destruction of Seoul would be accomplished within 4 hours of the Norks letting all of their artillery off the leash. You really think they wont be employing NBC weapons via arty?
see “atomic conspiracy” post, most of the tubed artillery will be at the edge of the range. NBC is a big concern, but it is not range specific to Seoul, their ballistic missiles can reach many places.
If they do employ NBC they got to realize that then even Chinese will not save them, South Koreans will then have no other choice but to try and take over the whole North and China will be shackled with images of chemical weapons devastation of Seoul and will not act to help them. I think the only targets that they will use non conventional stuff against will be US bases in Korea and Japan, perhaps Japanese cities.
Is the NORK munitions stuff Chinese (which might be pretty good) or homegrown (which is likely pretty lousy)?