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.
You are entirely right. These next few years are the most crucial in dealing with the North. They still have the conventional force to deal serious chaos to the south, and they have nuclear weapons (without any concern of the MAD concept). In another decade, their military will be little threat to the south (artillery aside), and the nukes will be expensive lawn ornaments.
Bush depriving them of their ill bargained for oil is great, in that it makes them less combat ready. While is most certainly is insane, he is not yet suicidally so. Bush has essentailly told Kim to go put a fork in himself, because we're done negotiating. This leaves it upon the North to show signs that they are ready to play well with others. Aside from a straight up public qui pro quo nuclear blackmail scare, no one cares what they say anymore.