Couple of quick questions after toying with AA BA and CX codes on LHR-LAX and JFK-HKG flights respectively.
1. Does CX have lower fuel surcharges than AA? Was pricing out the DONE4 and using the AA code instead of BA (on BA metal) on LHR-LAX reduces the surcharges by 100$ or so while using the AA code instead of CX (on CX metal) on JFK-HKG works out to be 100$ or so dearer.
Yes, CX has lower YQ than AA on that flight. AA booked me on their codeshare HKG-JFK even after I said the specific CX flight number 
2. I know single flight numbers count as one segment (eg. SYD-SIN-LHR). Does that also apply if the two sections of that flight are flown on different days? (eg. SYD-SIN on one day and the same flight number SIN-LHR on the next day?)
No, you may not. These flights, such as QF32, appear on the ticket as a single flight, and your booking is for the flight on that day with that flight number. Also, all connections over 24 hours are considered stopovers