What are the chances that we've missed a lot more than we've intercepted (think WOD)?
Rabid dogs are shot, not negotiated with. Time is growing shorter.
North Korea (nK) can't afford an overt nuclear program, financially or diplomatically. The threat of one is more beneficial than actually having one. Covertly experimenting with one, on a small scale, is the only realistic option. The threat really is not an arms race with nK, but the proliferation of nuclear technology, along with nK's missile exportation.
Regime change in nK is the only answer in the long term. How Russia, China, Japan, South Korea and US manage the mini-Kim's survival/collapse, without the deaths of several 10,000s is the tough question.