Seoul, and cities generally, will only fall if there are troops on the ground to take it. Bombardment, even heavy bombardment, cannot militarily take a city - just like London didn’t “fall” during the Blitz and German cities didn’t “fall” during Allied bombing campaigns. The civilian population in Seoul might be evacuated and much of the city seriously damaged in initial NK artillery bombardment, but only NK ground troops can actually seize and occupy it. And the likelihood of them being able to do so is very small at present.
In my hypothetical, I said that there could be undiscovered tunnels. Let’s triple the known ones and say that there are 12 undiscovered tunnels. Once tanks, trucks, and troops start pouring through, how long in terms of hours do you think the SK entrances would be free from air strikes designed to obstruct the southern exits and ultimately block the tunnels? To remain undiscovered so far, the tunnels would likely be fairly narrow and could not likely allow two or more tanks or military trucks to traverse the tunnel side-by-side. Just look at the ones that have been discovered. They don’t even look big enough for any kind of vehicle except maybe a motorcycle or a smart car. That is going to severely limit how many vehicles (if any) and troops can exit the tunnels before their southern exits are destroyed or collapsed. Massive artillery barrages and a few light infantry divisions that might be able to emerge from the tunnels before they are destroyed aren’t going to be enough to take Seoul, let alone South Korea.
Just like the main highway from Kuwait to Iraq was a shooting gallery for U.S. and Coalition air power, I’m sure the USFK/UN Command in charge of the defense of SK would fantasize about NK being so carefree and foolish to use open highways to fully advertise the presence and route of their invasion forces.
Vietnam was very different from the Korean War. And would still be very different from any Korean War II, since the government of South Korea isn’t a changing array of military dictators or strongmen that don’t have much popular support or legitimacy. The U.S. consistently beat the People’s Army of Vietnam’s offensives (greatly aided by its air supremacy over South Vietnam), but withdrew for mostly political reasons.
And MacArthur definitely did not do everything possible to avoid drawing China into the conflict. Even before China intervened, he wanted to take the war into China to overthrow Mao. He, along with President Truman, discounted plenty of warning signs and arrogantly assumed that even if China did intervene, U.S. and U.N. forces could easily handle them.