That approach does make more sense than a Banzai charge on Seoul, but I still maintain that the limit of the advance will be short of Seoul, or even past Seoul, or much further than Chuncheon at the most, before the supply lines are cut and the advance bogs down.
The idea of detonating a nuke to clear the DMZ is pure supposition, and would indeed cause problems with irradiating the supply lines...however, this is a country that bases its value of human life at about the same level that Joseph Stalin once did...so quite frankly I could see DPRK soldiers being forced to drive through irradiated ground, just to get supplies to the front. They'd probably wash the trucks on the other side at a small FOB type area, but the drivers would have their lives cut in half, but what does it matter? There are plenty more where that came from!
Mines are an old concept that I admit I had discarded, although one would need a large amount of high explosives to detonate an area large enough to cover the DMZ...but I guess it's entirely possible.