The question is : who will be holding the active flight records... CX or AA?

Wouldn't it be easier for CX to take over the booking entirely then to issue the ticket? I don't know if CX will be inputting the flights in as passive or booking it as active and then asking AA (or not) to cancel their bookings.

CX I think will still need to price the ticket through their fare desk especially if the tickets are to be booked on their ticket stock. CX may be able to 'see' your AA booking but they are the ones doing the ticketing. So their rules apply and CX has been known to interpret RTW fare rules very differently from their oneworld counterparts. The fare may be the same but the taxes won't as CX's CRS uses different exchange rates.. not by much anyway.