I don't think it is naive at all. If say Iraq nuked India, or Pakistan was nuked by India, (hypothetically) Or lets say Englans was nuked.
Don't you think they have the right to respond in kind? I sure as hell know that if I were attacked with a 9mm, I would use my 45. Or as they say in the cop world, you don't have to start at the bottom of the force contiuum.
As for China and Russia, they do not have the means to carry a sustained conflict with us, certainly by retaliating against us for exercising our right to self defense and preservation. Nor would the rest of the world
As for South Korea? What really do they offer that we can't obtain from the Philippines or elsewhere.
China at the moment does not seem interested in supporting N. Korea in its current dilema. In fact they have condemed them for what they are doing.
Hmmm, why use conventional weapons and deplete our reserves, on one 50 megaton on downtown N. Korea would solve the whole think in 1 second. The fallout would not be as bad as you predict, it would still be messy, but the hydrogen bomb is a fairly clean bomb and 99% of its lethal radiation is released at detonation and passes through everything. What is left is dust and such that may or may not settle and cause problems elsewhere. Didn't seem to matter when we were testing these things.