From my experience so far, when it comes to check-in for flights from Europe to China, they report the whole itinerary to China and wait for an answer before they print out the boarding pass.

Now the question remains if custom/immigration also receives that information in their system or not.

​​​​In PVG I have been told/warned by immigration to not change the booking afterwards, but I would of course make both reservations before leaving Europe.

Found another BUD-NGB-PVG-PUS flight with MU for less than 300 EUR o/w.

Even O/W flights to China are insanely cheap at the moment.

Azerbaijan Airlines takes the cake most of the times. E. g. two days ago they sold SAW-GYD--PEK for 144 EUR. Three days before christmas I paid 75 EUR for PEK-GYD, incl. 10 kg hand luggage....but since yesterday they do not offer hand luggage only fares anymore.