The Chinese will take Mongolia back in no time.
Yes. Han Chinese is an ethnic group. O3. Nough said.
Mongolia being part of China may be something inevitable. As Loz4en said, Mongolia is sitting upon a large treasure chest of resources. Mongolia has barely 3 million people (half the population size of Singapore) to fend for a country more than 2220 times the size of Singapore.
Either that or Mongolia would be a very under-developed country - I mean, what's the most you can do with 3 million people?
China has the population size nearly 450 times that of Mongolia. China wants the resources, she wants the land, she wants the tourism and other beneficiaries that she would receive if Mongolia is acceded to China.
If China takes Mongolia by force, it would be far less than peaceful though. Mongolia will practically become a 2nd Xinjiang where people create racial tensions and riots. Neither would the United Nations allow such a thing to happen.