GMD / KMT actually still contests the sovereignty of Mongolia and blocked their entry into the UN while holding the China seat. As for declaring independence, the East Turkestan Republic, which held parts of Xinjiang, was similarly independent, but the Soviets backstabbed their leadership and handed the territory to the Chinese.
Treating Chinese territorial integrity as inviolate when China has negotiated border settlements with various countries is complete garbage and gives people the sense you're an ultranationalist.
Point is, though, despite the fact that China will negotiate borders, it has no reason to with India when India, first, is actually expansionistic (annexed Goa, annexed Sikkim), and second, is busy aligning with an enemy state.