Is a stopover flight to Country C possible, if the stopover is not in China?

Unfortunately this is not possible. The key rule of any visa free transit scheme says that the immediate points before and after mainland China have to be in two different countries or eligible territories and your itinerary has the same country for both immediate points - Malaysia.

In other words, you will have to obtain a Chinese visa in advance or adapt your itinerary to comply with visa free transit rule. I'm sorry.