Hi MaxVO
As it happens, I am doing an all Y routing tonight, PEK-DOH-EDI. Not quite the 14 hours you will be doing, but same metal 77W & 788.
I have done one return trip on the same route in the last year and survived. Let's see what tonight brings????
Seating wise, I opted for a window on the 77W and an aisle on the 788 on the outward and on the return managed to get a bulkhead aisle on the 77W & bulhead window on the 788. Apart from the seats being narrow on the 788, I didn't find it too bad at 6ft 3in but was lucky in that I had the seat next to me spare on both the 788 flights
I have to order a GFSPML and thought it was quite tasty. Portion sizes are a little small but that's Y.
You don't mention if you have any One World/QR status. If you do then I believe that you can choose 'preferred seating' in Y, being the first three rows on a first come first served basis. I was lucky in my previous three out of four flights in that the seat next to me had been proactively blocked, which I am led to believe is something that QR do if loads permit for status pax in Y.
Hope that has given you some idea of what to expect. No doubt the QR experts will be along shortly.
Good luck.
Safe & Happy Travels
S