"I'm not for China establishing themselves inside North Korea and dictating a reunification where the reunfied Korea becomes a vassal state within a Chinese sphere of influence and neutralized as an American partner. I think the North and South Koreans would resist that as well. If China invaded North Korea and it was apparent that North Koreans were dying in large numbers to defend their homeland, I expect the South Korean would come to their defense, and instead of averting the showdown with China, we just arrive at it in a more circuitous manner."
I too, am very wary of a Chinese incursion into the North. There is no possible win scenario for us, the Japanese, the Taiwanese or the South Koreans.
You are absolutley correct in asserting that this sort of thing would allow Beijing to considate her power over the Peninsula and the rest of Asia.
Though I do not fully buy into the ROK coming to the North's 'rescue'. I think the Chinese will have a plan to destabilize Seoul or will negotiate a back door deal. Which, of course, they could renege on, at a later date.
A unified Korea under Chinese terms would be a disaster for the U.S. and our two major allies (Japan and Taiwan). But I think the Chinese know, short of going nuclear, there is almost nothing we could do about it, in the near term. We may be wishing that Japan had re-militarized a couple decades ago.
North Korea, up to a couple weeks ago, was acting like a crazy guy waving a gun around while surrounded by cops. They were mobilizing to a certain extent and alarming the neighbors with their missile exercises and nuclear "sea of fire" threats. (Maybe) China is the cop that decided enough was enough, stepped in, and put the gun to their head.
If I were talking to a North Korean military or intelligence officer (or the "Dear Leader"), I'd say that the nuclear and missile brinkmanship your country has been pursuing has reached a dead end and is now making a bad situation even worse as all your immediate neighbors are now talking about intervention. China is lining up on the border, Japan is talking preemption, and South Korea has leaked a military plan to move north and reunify - - all in the span of the past 2-3 weeks.
And what provoked all this? National maneuvers with nuclear-capable missiles.
Some deterrent.
Self reliance? Hardly. Self destruction? Definitely.
It's a dog-eat-dog world and you're the one wearing milk-bone underwear.
(That probably doesn't translate well, but you get the idea.)
The nuclear and missile programs are not your salvation. They have become your undoing. They are not deterring intervention but forcing it. Get rid of it. Put down the gun and don't commit "suicide by cop". Invite a UN force of acceptable neutral nations to oversee and verify the dismantling of these programs and simultaneously they would serve as de facto "human shields" from intervention. The UN could put a humanitarian assistance force together like they did for Albania in the mid-90s. No one is going to intervene with a multinational UN force on the ground.
Think about it.