North Korea seems determined to provoke the USA into attacking them.
My current theory about this is that China is behind it, and will protect NK so that NK can overtake South Korea.. and then China will have all of Korea as their puppet to antagonize Japan and Guam.
I also think that North Korea and China are in for a big surprise and will wish they had stuck to eating dogs and cats.You're theory has some flaws.
China is the worlds biggest economy and relies upon both Japan and the US to manufacture there while, at the same time, Japan and the US consume Chinese made products. Any attack against either the US or Japan would destabilize China's economy and all their development would come to a halt. South Korea is further down the list of strong economies and unifying with the North, with all the expenses required in developing the North, would also pull that economy down. In short, what you're suggesting is that China wants to destroy both itself and the Asian economic miracle.
First of all… the one's to blame for the disaster in North Korea is North Korea. They screwed themselves.
As for who is the biggest economy, that would be the United States which is 24.32% of the world's economy. China is a distant second at 14.84% Considering that China has over 4 times the population of the USA... that is pretty pathetic! Per capita, China only has about 3.7% of the world's economy.
https://www.weforum.org/agenda/2017/03/worlds-biggest-economies-in-2017/