Would having Sun Yat Sen live longer make any difference? If he could ensure a proper successor or at least establish a more dedicated cadre devoted toward his vision of China.
Sun was in favor of CPC-KMT cooperation, and was trying his best to absorb Communists into his own KMT so as to strengthen his Nationalist army to carry out the Northern Expedition. He himself may perhaps be blessed by thr Communist International after some time

The Communist Party did not even have to exist at all!
And Given his past records, Sun would alternatively be the first to collaborate with Japan, before Wang Jingwei and anybody else. He may simply sell out the Northeast in exchange of peace. If Tsuyoshi Inukai still becomes Prime Minister of Japan and was not assassinated, we may not even see a Sino-Japanese war since Sun would be willing to sell out national interests for the sake of his so-called ideals. Also, Sun and Inukai were really good friends in OTL.
I would still tend to think that the latest most likely POD for a KMT victory would be no Xi'an Incident, assuming that the Sino-Japanese War goes similarly. Nationalist forces had almost won the civil war by then. Had Chang Hsueh-iang, the worst traitor not staged the coup, Chiang would have won by March or April 1937. However, you still have the Northeast Anti-Japanese United Army in Manchuria. It was in name under CPC control, but was indeed directly receiving orders from Moscow. After the war, you see a Communist Manchuria and Nationalist elsewhere in China. It's likely that the KMT would win, but a permanent split would remain possible, as far as Red Manchuria would be receiving the weapons of the surrendered Japanese.
If you put the POD after 1944, I would say the KMT could at best get the Southern half of China, unless Chiang did not sack Sun Li-Jen in 1947, or that Dai Li somehow survived. Nevertheless, it's 200% ASB for China to regain Vladivostok. Why would the Soviet Union returns it to China, when it has been under Russian control for that long? The United States would never be able to ask the Soviet Union to gift it's territories away. That's just ridiculous. Chiang would after all invade Tibet, and he did have such plans. As for Mongolia, Chiang did recognize it after 1945 in OTL, though he would claim Mongolia once he fled mainland China to Taiwan

I honestly don't see how you could get Mongolia back. Even if a referendum is held there, I don't find it likely that Mongolia will come back.