Securing Chinese cooperation is the key to cutting this particular Gordian knot. As the only country with notable foreign influence in North Korea it's probably the only one that could hold leverage over them, the only ones that could execute a coup with any chance of success, and if anybody was contemplating a military solution, an invasion from China would circumvent essentially all of their 50 years of military preparations.
North Korea may actually seem less threatening in 15-20 years than it does now, because while it may climb to a nuclear arsenal of 50-100 and have intercontinental range, technologically speaking advances in BMD and CRAM (assuming that's the path Japan and South Korea want to go) could more or less neutralize this as a serious deterrent.