Any such artillery employment is going to be in the face of nearly immediate ROK/US air supremacy, including 24/7 armed drone surveillance on station. 30 minutes into the war they won't have any major stationary launch sites left and probably no airfields either. Mobile launches are visible from space. Nowhere to hide.
I wouldn't try it, but then I'm more or less sane. Dumber things have been done, I suppose...
“They” can hit Seoul to the extent that their Koksans can hit Seoul. They have under 500 of those. Slow rate of fire, unreliable, difficult to avoid return fire.
Shelling a city is anything but “each hit takes down a building”. Look at how long it took Russia to flatten Grozny; cities don’t go down that fast. And with a mile or two of error, the most likely thing to hit in Seoul is a single-family house (check out satellite images of the city - they cover a much larger portion of the city than skyscrapers). Quite possibly not even that - roads, trees, parks, sports fields, rail lines, into the Han, etc.
If they shell Seoul, it’s going to be a brief terror attack that’s going to cost them dearly.