Sorry to say the scenario you paint is completely unrealistic. The Western Allies didn't care much about Manchuria, and North China. They did care about Eastern Europe, and it took 45 years of Cold War to get the Soviets to leave. The USA did get the Soviets to leave Northern Iran in 1946, because it wasn't that important to them, compared to the Far East. In 1945 Nationalist China was a fragile State, devastated by war, militarily, and economically weak. The Chinese Nationalists were hoping to get what aid they could get from West, and East, and hang on for deer life. The Soviets for their part were striping out the industrial plant in Manchuria, and shipping it back home, they weren't about to stop, and put it back.
As for an aging Stalin backing down from a threat of war from Chiang the idea isn't serious. The only danger to Stalin would be an early stroke from fits of laughter. Like Churchill Stalin regarded Chiang, and Chinese Power with contempt. Stalin was a ruthless, and evil cynic, only power entered into his calculations, and Nationalist China had almost none.
China had much more to lose than the Soviets, and conversely what would the Soviets have gained from fighting China? Booted out of the UN and alienated on the world stage?