As I mentioned in the other thread, the problem is that there is a huge artillery army dug into the side of a mountain 50km away from central Seoul. Experts have deemed this massive force "nuke proof" and basically nothing can be done to prevent them from raining hell down on Seoul. Some experts have predicted this force could level Seoul in only 2 hours. Furthermore they could lob nukes into Seoul from this location. Even if we are wrong about this, is it worth the risk?
So ofcourse the US could destroy NK in a week, but Seoul would go down with them. As a minimum Seoul would have to be abandoned first.
As strange as this sounds, the best way to deal with NK is to "cut a deal" (Trump reference not intended) and force their leadership to open up the country in exchange for their lives. They know more than anyone they won't survive a US assault, even in a fantasy scenario that includes China getting involved. Let them live in exile in China and die happy.