You don't seem to understand where Incheon is then.
View attachment 109593
From if you draw a line from Jeju to Shanghai, everything within can get hit by even Chinese old gen anti ship and cruise missiles.
So what is the point of "commanding ROKN forces at Incheon to blockade any Chinese forces from coming out of the Bohai sea"? You might as well say "China will command forces stationed at Solomon islands to blockade all US navy coming from Hawaii".
Incheon is not defensible. It's not "north Korean coastal raids" they're going to fear, it's mass bombardment by NK and precision strike (likely in numerically no less terms than what NK brings) by China, followed by all out land assault.
Busan is probably the safest area in SK and it is still not safe from strikes.
From anywhere around the Bohai on the CN/NK side, DF17s can hit all of SK. This means you can't even base a ship in a SK harbor for longer times as it'll get striked by non interceptable.
SK ground forces might be as fanatical as NK ones, but if Seoul participates in an invasion of China, it'll not just be NK hitting them, but they'll be surrounded by Chinese strike options and if not Russian direct participation, then at least Russian ISR assets freely flying over "international" airspace and lighting them up.
It'll by no means be a trivial fight because of their fanaticism, but China has a ton of North Korean bodies, and with cut supply and destroyed infrastructure, the odds are very heavily stacked against long term SK survival.
Of course America in the event of SK agreeing to an attack on China would take over the command, but they would still need to use their pawns smartly, and that means keeping them at most around Busan and the Tsushima straits.