"1. If I change the FF number on the reservation now to my brand new AA FF number, is there a downside? For example, currently when a QF boarding pass is issued to me it shows my QP membership. Obviously if the AA number is in the system this will not happen. Can I still access the QP by showing my QP membership card and boarding pass?"

In the words of Ansett. Absolutely .

No problems there, just show your QP card and in you go

"2. Would I be better off getting boarding passes with the QF FF info on them and getting the staff in the QP to change the info in the computer?"

There is a lot more chance of things going wrong and points getting credited to the wrong location. QF staff make enough errors at putting AA numbers in without adding this to it. I recommend, phoning QF and getting the QF number removed from the booking. Use checkmytrip to ensure that it is removed. Then call again and ask to add the AA number. If correctly entered, it will appear on checkmytrip as

Qantas Airways - AAxxxxxx

The Qantas Airways words in front are perfectly normal. If it appears in any other form, phone and get it removed and then re-added

"I really don't want to risk being refused entry to the QPs in CBR, SYD and LAX. On the other hand, by my calculation my CBR-SYD-LAX-IAD flights (booked in H class) will just get me to the required 10,000 Q-points, so I can't afford to make a mistake."

Looks right to me

CBR-SYD - 500 qpoints / 500 miles
SYD-LAX - 7491 qpoints / 7491 miles
LAX-IAD - 2288 qpoints / 2288 * 2 miles

Are you travelling on the AA flight number or a QF codeshare to IAD? If on an AA number, I would phone and check with AA that your membership number has filtered across

Total qpoints earned = 10279 so just nicely over the threshold giving a 2288 bonus on the flight for completing the challenge. Once your status appears on AA.com, you may wish to remove and readd your FF number into your AA bookings so that your AA boarding passes show you as platinum , allowing you to use the elite security lanes at LAX et al plus getting Group 1 boarding. Also, if you wish to pay the extra $25 per 500 miles, you can then also request upgrades for any other AA domestic sectors you may have

Dave