In theory, if Japan doesn't wage war in the Pacific AND the situation in China stalemated AND later communists won the struggle for power in China we could get the Soviets-PRC-Manchukuo configuration holding for some time before the Sino-Soviet split. Or is that too farfetched?
I don't think this could happen, since once Japan expanded beyond Manchuria they were set on a course of iterative expansion that made the Pacific War hard to avoid. On the other hand if Japan
doesn't expand beyond Manchuria (i.e. freeze their expansion at 1931), the conditions for the CCP to overthrow the KMT don't exist, since the KMT would not be fatally weakened by the war of 1937-45 (which hurt the KMT far worse than the CCP).
The most likely outcome would be probably the KMT would maintain a tactical alliance with the CCP and eventually (say sometime in the 1940s, once they had built up Chinese strength sufficiently) launch a war to force Japan out of Manchuria. In such a case Stalin would probably be mainly concerned to protect Russian interests in the
Manchurian railways (a Tsarist hold-over), rather than promoting the interests of the CCP.