The Chinese know enough not to get involved in Korea in that way. These 2 cultures have thousand years of exchanges, good and bad.
Yes, China has always wanted Korea to be united. But just not under the US-bed partner South Koreans.
The geo-political situation on the Russian border is slightly different. Russia and China had border clashes, nothing more serious.
OTOH the Korean peninsula is a powder keg, and it is also the doorway for bad things to happen. On that side China faces ROK, Japan, Taiwan and worse of all, USA.
Bad tempers or Mongolian bloodline did not prevent the Koreans from being colonized over and over and over throughout history. Sorry, no one is intimidated by bad tempers. Mongolia, as you already know, is nicely carved up into pieces.
Contrary to what you said, the Chinese have always treated the Koreans as a close relative - the North Koreans that is. There are ethnic Koreans on the Chinese side of the border for generations. (And vice versa.) They have not faced persecution or discrimination. Chinese and Korean cultures are very similar and both are Confucian-based.