How can this happen? If he rebels, then the Japs remove him easily.
If he does it after Japan is kicked out, then everyone in China will see him as an opportunist collaborator and he will either get arrested or at least told to shut the fuck up.
It doesnt help that no one wanted the monarchy back in China neither Mao or Chiang. Neither would it help either side.
An emperor is a hardcore no-no in communism and would only serve Mao to lose support among communists.
If Chiang used him it would only rally more support to Mao.
You would need to somehow have Puyi being capable of providing significant resistance to the Japs in Manchuko and be capable of raising an army. The details of how he can accomplish this I will not explore since it is kinda ASBish to imagine how can a popularily deposed puppet emperor of a brutal occupier can do that in occupied territory.
But assuming he did I imagine he can get to play a role in the civil war or at least become a mouthpiece against communism and hope Chiang wins. But again, this will likely just make Mao even more popular, unless he built insane respect from his former resistance to the Japs.
If Chiang wins, maybe he gets to rule a limited area as a puppet emperor of the Republic of China, but more likely he will only get the terms his mom negotiated for his abdication respected (that he would still be treated as emperor)
In every scenario where Mao wins, he gets screwed.
I informed myself further and it seems he was even more screwed with Chiang, who was out to get him. Mao was surprisingly extremely merciful to him. It seems the guy honestly had no hope to survive as Emperor.