The big story I think many are missing is - what happens if the US+SK manages to completely flip NK into a pro-western nation.
The talks are already moving at a pace very few predicted. China has already been bypassed. It might have been a crazy theory a month ago, not anymore. The momentum is there.
An increasingly strained US-China relationship put NK in a similar position to that which SK was in around the Korean War: a poor country that could be propped up by Western regimes not only as a quid-pro-quo for military alignment, but an example of what you get when you align yourself with the US (see also: Japan, Gulf states, Germany). NK's nuclear weapons give them a bargaining power that may be strong enough for the current leadership to remain in power or at least unharmed. Chasing the kind of wild economic success that SK experienced when propped up by the US is a no-brainer
Nobody over there wants to be China's puppet state, that will include North Korea if it decides to open up. That doesn't mean China won't have an outsized influence in trade and politics (it obviously will given its immense economy), however NK will very obviously aggressively seek to diversify its trading and become less dependent on China.
The US is about to make Vietnam rich. As it did with Japan, South Korea, and China before it. NK will seek the same exact outcome.
US imports from Vietnam, 2017: $46 billion
To put that last figure into context, that's equal to the US imports from France, which has a $2.5 trillion economy (~22 times larger than Vietnam), and is nearly equal to the imports from the UK.
Extrapolate that forward, with the assumption of a slowed rate of growth. It's the exact same way the US made Japan and South Korea rich, through massive imports, capital investment, and the corresponding industrial build-out.
US imports from Vietnam will be close to equal to 25% of their entire economy this year. Vietnam's present economic boom, is almost exclusively due to the huge boom in US imports. The US is building out their industrial capability, which will spur further build-out and advancement, and on it will cycle.
China is an economic competitor with Vietnam. They of course also have a long history of not liking each other. The development of Vietnam makes both practical sense for the US in terms of manufacturing costs, and strategic sense to build-up local rivals to China (or more generally strengthening the region against a Chinese hegemony in Asia). All the US has to do is point some of its import demand away from China and at Vietnam or other local nations. China's manufacturing costs are far higher now than they used to be, they're non-competitive in many things now on cost. Vietnam has even begun to steal away large volumes of tech manufacturing, for eg smartphones.
It's further worth noting, the trade with Vietnam is about as one-way as it gets (and note that the US isn't complaining about it, that's because it's strategically desirable for now to rapidly advance Vietnam). Vietnam only imported $8.1 billion worth of US goods in 2017. So Vietnam is being showered with a flood of dollars. That will serve two outcomes for the US: it continues to bolster the US dollar's global role, and Vietnam can purchase US treasuries with their mountain of dollars, as Japan and China did and for the exact same reason.
China imports from Vietnam, 2003: $1.63 billion [0] China imports from Vietnam, 2016: $35.6 billion [1]
Seems in the same ballpark area to me as the US numbers.
Your theory that "Vietnam's present economic boom, is almost exclusively due to the huge boom in US imports" doesn't check out with these numbers.
[0] http://atlas.cid.harvard.edu/explore/?country=239&partner=un... [1] http://atlas.cid.harvard.edu/explore/?country=239&partner=un...
Regardless, I'm not China, and I'm not commenting on where the blame should be sheeted home; I am just commenting on the geopolitical instability of the whole Korean peninsula being dominated by a rival great power to China (the same is true if it were to be entirely dominated by rival powers to Japan, incidentally).
I agree that Kim's preferred outcome is almost certainly to remain in control in NK. But it may not happen. If his regime falls due to internal pressures after denuclearization, why wouldn't we expect NK to join the West? Why would we expect Kim's regime to continue to survive after denuclearization?
The only way it could happen would be if he were completely pardoned, which is logical if it benefits the greater good at no further cost, but very against human nature.
In all likelihood, by his perception, he's done nothing wrong! So...it won't be something to discuss, for him. It would be a great offense and a threat.
It's happened in the past, e.g. with Pinochet — and later on the Spaniards went after him anyway, in spite of the amnesty and in spite of the bad example this would set for future dictators.
Kim knows that being a rich person in South Korea is mighty good. That may be his goal, but getting there won't be easy.