Who would step into his shoes and take command, and how would the Civil War play out thereafter?
I think the general consensus on a Chinese-Soviet union of any kind is that it's Sovietwank. Chinese communism was going in different ideological directions even before Maoism became a formalized thing, and it's doubtful the Soviets would have had the administrative ability to handle China. Even beyond all that, the Chinese were and are a fiercely independent people and would never tolerate becoming another cog in a foreign empire.It's an interesting PoD. Unfortunately, I don't know enough to say who would be the next in line. It's possible, however, that whomever takes over the movement won't be as successful in taking over China. Whatever else you can say about Mao (and I would say virtually nothing good about him), he was willing to take big chances and make sweeping changes to society/the movement/etc.
If there is no strong communist movement in China, perhaps the USSR makes a client Chinese/Manchurian SSR?
and it's doubtful the Soviets would have had the administrative ability to handle China. Even beyond all that, the Chinese were and are a fiercely independent people and would never tolerate becoming another cog in a foreign empire.
And regardless, a Sino-Soviet split would have occurred regardless after the formation of the PRC.