1. You can find op-up priority in the FAQ section 8 here.

If you have credentials from multiple airline programs, and you are flying CX, and you are hoping for an op-up, you will definitely want to use your MPO number.

2. Most evidence points strictly to BN used to resolve op-up conflicts, with no consideration for fare paid.

3. Agreed, mileage upgrades beat op-ups. Even last minute mileage upgrades at the gate beat op-ups.

4. Make your booking first then call AM and request the upgrade. You can also try the online upgrade thing but I've never gotten that to work. Calling earlier should get you a better spot on the waitlist (within your MPO elite level).