Unlimited stopovers are still permitted from Spain and there are no date restrictions on the fare so you can (in theory) do this in December.
I'm going in March. My flights PEK-PVG, PVG-XIY and XIY-HKG are all on MU booked in J class. However, most of the MU flights don't have J available, only F, which isn't permitted. Conversely, CZ has J available but that isn't permitted. XIY-HKG is possible on KA but wasn't available when I needed.
In summary, it's a difficult itinerary to build so you need to:
- subscribe to Expertflyer
- study the routing rules carefully
- find the right availability on EF for your dates
- input those flights into Matrix to price it up
- then find somewhere to book it.
Good luck!
