Define "disintegrate". Does the KMT blip out of existence, are they beaten by the Japanese? The Communists? Russia??
I think the likeliest scenario (assuming they just blip out of existence and there's a big power vacuum) is the CPC takes over. Communism, albeit in a million different forms, was very popular among the peasantry because of land reform promises and the KMT's infamy from dealing with western capital. The soviets can't realistically just annex large parts of China, plus they're still backing the CPC. I also believe the KMT holding on in south china to be the best bet for an american-backed china. There's no way the US lands a boatload of troops, in the middle of their busy campaign against Japan, to restore order in China or to fight of the CPC who at this point aren't anti-American yet.