Ultimately though if the rosy situation described by Swerve was a reality, then how would North Korea have been able to get away with its behaviour for so long? It is simply because it knows that any attempt to attack it would drag in all the regional players and so it trades on it....
No, it's much simpler than that. North Korea is dangerous. A war would be horribly bloody, & would cost most of its neighbours a vast amount.
You have things about-face. Nobody has any interest in attacking North Korea. What they fear is North Korea starting a war. It gets away with so much by, essentially, blackmail. It's like someone with a suicide bombers belt strapped around him, demanding food, money, etc. from all around him. Nobody dares shoot him, for fear of setting off the bomb, & nobody dares deny his demands, in case he sets it off himself.
North Koreas forces lack the logistics for a sustained war & its weapons are mostly museum pieces, but it has so many troops, so many guns, mortars, & rocket launchers, so many tunnels & caverns to hide them in, and so much of S. Koreas population & industry is close to the border, that N. Korea could kill vast numbers of S. Koreans, & do immense economic damage, before being beaten.
The military defeat of N. Korea would bring about a complete economic collapse of the country, & probably trigger the flight of millions of refugees. The logical direction for them to flee would be towards China, causing a huge military, economic & social problem for China. Disruption of trade caused by a war would also be extremely expensive.
Chinas response is complicated by internal factors. There are millions of Korean-Chinese on the Chinese side of the border. So far, they prefer life as loyal citizens of China to the alternatives. China does not want to arouse nationalist sentiments among them.
You referred elsewhere to China preferring re-unification under its proxy. That implies some false assumptions. Firstly, that N. Korea is a Chinese proxy. In reality, while China may enjoy seeing it cocking a snook at the USA, it is far more of an irritant than an ally. N. Korea is expensive for China, in aid to keep its economy functioning enough to keep the number of starving Koreans trying to cross the border down to a manageable level. The border needs guarding closely, both to keep out refugees, & because N. Korea is badly behaved enough that its border guards will pursue those fleeing into China, if not prevented by Chinese guards. It is unpredictable, which is very worrying in a heavily-armed neighbour. It is economically incompetent, making it uselss as a trading partner. If it had the industry (damaged, but most would survive) & technology of the South, it would doubtless feel emboldened, & even less likely to be a good neighbour to China. N. Korea needs external enemies to maintain its system. Take away a US-backed S. Korea, & what is left? Japan & China. Japan is across the sea, & N. Korean claims to islands could be no more than an irritant to it. Japan will always be able to deal with them rather easily.
Once the S. Korean economy had been reduced to the catastrophic level of the north (& it would be), it would be back to blackmail for essentials, & China would probably be a better blackmail target than Japan - remember that Korean minority across the border in China, & the land border.
For S. Korea, there is no incentive to attack the north. Assuming it would win the bloody & destructive war, it would still have suffered immensely, & would then have the ghastly prospect of trying to make good the damage, at the same time as integrating the disaster area that is N. Korea. An appalling prospect.
Things aren't rosy, & I have never said that they are. N. Korea is a sore in the side of East Asia. Where we differ is that I assume that N. Koreas neighbours will probably seek to act in their own best interests, while you assume that they will all act irrationally. They may, & one should consider how to respond if they do, but it does not make sense to assume that irrationality is the most probable behaviour. Nor does it make sense to assume that they will act against past precedent - and remember that Russia has always avoided directly fighting the USA, as has China since the end of the Korean War.
