The deal is this. After decades of near (military) parity with the South, the North is in free fall. Their factories can churn out all the spare T-55, and T-62 parts they want, but on the whole, the KPA is showing its age, and has no way to modernize. The ROKs, on the other hand, are lean, powerful, and high tech. Every year that goes by, the invasion option loses some of its menace.
Enter the bomb. It is only useful against the U.S. and Japan (Kim would be the most hated Korean in all of their history, and for all time to come, for destroying Seoul. In a culture that is obsessive about 'losing face', that is not in the cards) To deliver this bomb to us, Kim needs missiles.
Enter the ABM defense system (especially the air to air laser platform the USAF is working on, for boost phase intercept). See where this is going? Once we rob Kim of his ability to deliver, he'll be left high and dry.
The NK problem is unique, we'll just need to think outside the box to get these particular bastards.
Yeah, OK, I see that. But there are two problems in reliance on a defense shield (three actually, but one isn't related to ABM defense): 1. The shield has to be highly effective against even one single missile -- no mistakes or "tolerances" allowed; 2. We need to hope that Kim doesn't go mad or wake up on the bad side of the bed for the few years left necessary to implement and perfect the ABM system. That's a lot of time for this situation to continue at what is in effect status quo. A lot of room for error there in dealing with a desperate madman.
The third problem is the assumption that Kim would not use a gravity bomb on Seoul. If he becomes desperate enough with the passage of time (particularly if that time is measured in years), does he really care about who in the South hates his guts? When you are an absolute tyrant you force people to "like" you. And usually the conqueror, the "unifyer", doesn't particularly care which weapons he employs to gain his goal.