What would happen if (unlikely) part of Myanmar now wanted to be part and integrated to China as one of the provinces ? Would China accept it ?
No way, Kokang is poor as hell and middle of nowhere. Pretty much the whole interior is the same, all the wealth (such as it is) is on the coast. But the coast is farthest away and least China-friendly part, so it's much better to have influence over the government which controls the coast. Also ports could be useful for strategic reasons.
Rumors only, but my uncle in Kunming told me that MNDAA asked to join China back in 2009 when they were desperate. But they are crazy ethnonationalists who don't follow orders. That's why UWSA are much closer to China even though they aren't Han, they know how to listen.