An important question here is whether total Nationalist victory occurs before or after the Sino-Japanese War. I suspect that if it occurs afterwards, the Nationalists may very well replace Tibet and Xinjiang with Communist North Korea and North Vietnam as the targets du jour; those areas were traditionally in the Chinese sphere of influence, and I doubt Chiang would like the idea of potential havens for ChiCom remnants surviving on his borders. Obviously, he can't do anything about the USSR, but the other bits would be more his speed.
Also, how might an independent Tibet and Xinjiang develop? Would the Soviets simply add Xinjiang to their Central Asian holdings?