Thanks for the great (video) analysis, and as you say always good to hear form others how they perceive this issue. Seems you are from a North-Korean/American background.
I totally agree with the 'buffer zone' principle you have mentioned. However, I have a different opinion as you might have guessed.
South-Korea is an independent country and my take is that many SK would love to see the US leave, for various reasons, but the most important being that the US is as big an obstacle to peace in the region as China is (Note: I am not saying that SK isn't thankful to the US, they should be).
The reason China is using NK as a buffer, is the same reason the US would never agree on leaving. Lack of trust by both China and US in eachother.
SK is one of the biggest trading partners of China and reunification would have big implications for trade, tourism, stability etc.. SK and China working on a mutual plan for unification of Korea is in my opnion not impossible, but the US would have to make way for this.
Why do I put the responsibility with the US? My take is that the US is the only party involved, that does not have a necessary geographical presence (i.e. the US does not have boundaries with Korea nor China). With that, the US has a responsibility to make way when peace can occur. But don't get me wrong, China has a big responsibility as well. Protecting SK and ditching the regime in the North to give independence to Korea as a reunified country.
Korea and the region would benefit from unification, it would create stability and it would solve the biggest humanitarian disaster that currently exists in the region. But as always, human rights play the second violin, right next to the worlds smallest violin, and it is currently playing for almost everyone except the people of NK.
Background: I am from the Netherlands and live in Hong Kong. My wife is Korean.