But on the other hand I should point out that after the Second Opium War, when Beijing was taken and the Summer Palace sacked, the Qing were still able to defeat the Nien and Taiping Rebellions. While it's not terribly easy to end the Qing Dynasty, it's not terribly hard to imagine their earlier fall either.
Perhaps you could feature an analogue to the Taiping Rebellion, only the rebels manage to seize the Grand Canal and move north into Beijing, and you could go even further by having them join up with an analogue of the Nien Rebellion. At the same time, I can't imagine the Manchu and Mongol populations going over the the alt-Taiping's side, so you might get a situation where the remaining Manchus and Mongols form a rump state in the north. Turkestan and Tibet might become independent if they want to during this time, though I don't know enough about them.
The good thing about peasant rebellions in China is that they can be very quirky. Sometimes rebel leaders declare themselves to be Buddhas, though another guy decided he was the younger brother of Jesus Christ. So you could probably invent a new, syncretic religion alongside a rebellion and get away with it.