Ok, one thing at a time.
Do the maths for FRA/HKG via PEK for 2 acclimatised flight crew
Two sector day, departing FRA; Unacclimatised FDP available is 12:15.
Sector time to PEK about 8 hours (I'm guessing). Add one hour for sign-on, add one hour for the turn around and three hours for the flight to HKG gives a total of 13 hours.
How is this do-able with 2 crew?
It's not, but it is just do-able with acclimatised crew (sign-on between 0800 and 1259), which is why so many Euro carriers do it.
It is legal, but a HKG crew could never be acclimatised, and a based crew would never be acclimatised in HKG.
Try working out LON-JNB...