I would be very surprised if the US has not already been discreetly discussing "What if?" with China. The young'un is overplaying his hand, so it must be for domestic consumption. I've read in the past that China's primary motivation in supporting NK is to prevent its collapse and the flooding of millions of starving Norks over the border. Today's China doesn't want to export revolution, they want to export business and acquire territory. Wars get in the way of business.
As for shipping containers, I find it hard to believe that the few of them coming out of NK are not totally monitored. NK has no economy, what could they be shipping? Maybe some arms or technology, and don't think those would not be of intense interest to western intelligence.