They don't have that many nukes. What nukes they have are low-power. If they hit us with nukes, then we shall retaliate with nukes.
Seoul is built up considerably from 1950. It is a megalopolis, with suburbs running almost all the way to the DMZ. Infantry advancing in human waves will be cut down by artillery and aircraft, as the Iranians learned in the 1980-1988 war. To advance, you need artillery and tanks - combined arms fighting. Seoul will be a defender's paradise.
The South Korean Army is very good and very professional. The Viet Cong feared them more than the US Army. The Korean Military Academy turns out first rate officers.
The North would run into a buzzsaw and would be mauled.
MAD only worked when both sides were rational and both sides have something to loose.
N. Korea is run by madmen and has virtually nothing to loose.
They do not need to advance in human waves, all they need to do is use a few nuclear weapons and follow that up by traditional artillery and some air bombing and naval operations.
The real question is would China invade N Korea to stop them prior to the USA doing anything dramatic. I think not. In fact I would wager if N Korea were to detonate a nuclear weapon on S Korea, that the next thing that would happen would be for China to warn the USA that it will respond with nuclear weapons to any nuclear weapons launched against N. Korea.
Until China is no longer willing to support N. Korea, N Korea will be able to do just about any military activity it wants and get a way with it. Look to history and read up on the USS Pueblo. The N Koreans are willing to start wars because their leaders are not afraid of having several hundred thousand of there people killed.
Ultimately China will need to take care of the N. Korean problem, because it is in their self interest. I can only hope that will be sooner rather than later.