Cathay Pacific Asia Miles - "open jaw" at destination for oneworld award?

Dear all,
I am wondering about oneworld award on asiamiles: Could I go AMS-HKG-NRT-LHR? since it is "miled" by total length i would assume I dont need to return to origin country unlike AM awards.

This way I could dodge the 80 quid APD...

At the same note, how is BA J compared with CX J (for LHR-HKG vs AMS-HKG)
What I know: BA- flat bed, sit forward, lounge
CX- ability to arrive before 0800, "better service"

I've confirmed with MPO- no problem ticketing with "open-jaw" on a oneworld award as it is essentially a one way ticket.
I'll just keep this here for documentation. the trip change is bogus as i would be in AMS either way. (LHR-HKG-KIX//ITM-HND//NRT-xAMS-LCY(orLHR))