Incorporating Manchuria into the USSR would just be stupid and everybody knew it. You'd have a resentful population far more loyal to Beijing than Moscow and whatever other gains there would be would be minimal or insignificant when compared to the general antipathy the populace would feel towards their Russian overlords. Mongolia is one thing, Manchuria is quite another.
Until 1945 Harbin, for instance, was a typical Russian city where Chinese were a minority. There were also Cossac settlements - so called Cossacks of Seven Rivers Valley. There were much more Civil War immigrants as well as the old Russian population settled there since the time of construction of Chinese Eastern Railway.The куые ща еру population was mainly indifferent to Beijing and could hail whoever from Puyi to Stalin. The point is that the Russian population of Manchuria was less inclined to submit to the Soviets.
What concerns Mongolia, USSR was cautious in that matter not to spoli relations both with Chiang Kai-shek and Mao until 1943 not finally choosing the person on whom to out their stakes. But as Chiang Kai-shek demonstrated his pro-American orientation, the only facceptible figure for the Soviets was Mao. However, irrespective of the political figure, China - any China, Communist or Nationalist alike - would strongly oppose any attempt of incorporation of Mongolia into a different country.