What do you suggest? More diplomacy? Clinton and Carter did that and look where that got us.
I'm not sure I understand your point. How does a nuclear Japan help things? If the interest is in deterring a Korean attack on Tokyo, the US could simply reassert its historic commitment to providing a nuclear umbrella. A nuclear Japan isn't any more likely to launch a preemptive attack on Pyongyang than a nuclear Washington is. In fact, it's probably less likely to.