1. US is not going to overthrow NK, I highly doubt US will even attack NK, simply because NK has China's indirectly backing so US would think twice before they try Libya or Iraq on NK. US still remember what happen in 1st Korean war, and they won't make the same mistake twice.
2. Even if US do attack NK (highly unlikely), it would at most, limited attack against NK nuclear capability with cruise missiles and bombs, no US or SK troop will ever set foot on NK soil. And no, if US/SK bomb NK, NK at most will rocket SK but NK will NOT invade SK because fat Kim know its suicide. And from SK point of view, I personally don't think they want to take over North as well, 1, SK is too developed and too rich to suffer such high casualty in a unification war. 2 SK don't want to stuck with such high reconstruction bill 3 SK need to secure China's approval to unify North Korea and I doubt they will get it without US leaving and SK is not going to ask US to leave.
3. With the above 2 point established, NK will not be defeated or regime change, theretofore there is no reason for China to help US with Korean unification, so any further talk of Chinese troop resistance level or buying off the Kim family is a moot point.
4. I think I have been saying from the past 5 pages or so, that Korea unification is NOT in the Chinese interest with or without US troop presence at Korea peninsula, just as it was a mistake for China to help Vietnam to unify, it will be equally bad idea for China to help Korea to unify, if you disagree please go back a few pages and see my posts.
Therefore John Bolton is just being John Bolton as usual, in fact that article is quite lazy, he literally want the cake (Korea unification) and eat it too (With Chinese help), I think I have higher chance of winning the lottery than what John Bolton wants.
What I think is going to end up happening is that Trump have to accept a ICBM capable North Korea, and then start to compromise and negotiation from there.