I'm no expert on geo-politics, so take any of what I am about to say as "some middle aged fat guy playing armchair general".
It's well known that NK has an enormous amount of artillery pointed directly at Seoul, SK, and I think it's a safe bet that should any kind of military strike be directed towards NK, those artillery units would be ordered to rain shells down on Seoul. No matter that there are loads of underground bunkers in Seoul, there would still be an enormous loss of life. Not to mention, with SK being the 7th? largest economy on the planet, and the fastest growing economy in Asia, not to mention very friendly with the US, the economic impact on the entire globe from Seoul being shelled is too big a risk.
I think, and this is just my opinion, that neither China nor Russia would be too bothered if the Kim regime was taken out. NK is kind of a PITA to everyone, and the flimsy trade agreements NK has with other countries are not worth the paper they are printed on. The only thing NK has to trade is coal. They lack the infrastructure, healthy manpower, and technical acumen to take advantage of the TRILLIONS of dollars worth of mineral wealth under their soil.
How to get rid of Kim? No idea. The logistics of an operation that would eliminate the upper echelon of leadership while preventing retalitory strikes against SK, Japan, or any other place the bottle rockets NK calls ICBMs can reach, is a logistical nightmare. The Navy Seals, Marine Corps Force Recon, Army Delta and Green Berets...they can only do so much.
Then the question becomes - what do you do with the NK people after the fact? It might take a couple of generations to undo the brainwashing those people have endured, and you can't just reunite the Korean peninsula at the snap of a finger. Who takes care of the people? Who goes in to help them? The US? No way...they are raised to HATE the "wicked imperialist US". So who? The UN? Nope...too much association with the US. The human-aid necessary to help the population is mind boggling. Food, clean water, medicine, help with agriculture, rebuilding infastructure, etc. Then what about the "elite" of NK society? Are they as culpable as the leadership regime for the problems in NK? What is done with them? Seize their financial assets? I don't have the answers to these questions.
The NK situation is particularly sticky. The whole world has the UN and the US political machine to thank for it. I don't know that I agree with Macarthur that nuclear weapons should have been used on NK, but I have to wonder if maybe seeing Pyongyang leveled under a mushroom cloud might have taken the fight out of the NK troops and given the Chinese something to think about seriously since they hadn't yet tested their own first generation weapons. It's all moot anyways to think about these things, but I wonder how things might have been different.