1&2 Answer can be found on the sticky FAQ

3 No. Asiamiles upgrade definitely beats op-up - after all, CX is taking something from you rather than giving upgrade for free.

4. While there is the option on cathaypacific.com under redemption, I am not sure can it progress upgrade redemption. Call AM / CX is ok.do it in the airport is also possible.
It is on a first come first served basis if seats are available - while waitlist will consider the tier first. If you do it at the airport, as long as there is empty seat on the flight, you should be able to redeem.