"To take the challenge do I have to have an AA account (likely) or can I do it on my QF account? "

You will indeed need to have an AA account and collect the miles for the trip into the AA account to do the challenge

"Is booking early worthwhile, with regard to getting a good seat, or should I hold off and get an AA account to use with the booking?"

You can get an AA account in minutes just by going to www.aa.com and signing up. As far as getting a good seat in WT+, well, earlier the better I would say.

"Getting the additional points would be good. I'm already a QP member, so can access the lounges at the moment, although can I still do that with an AA number of my boarding pass?"

With QC membership you are allowed to use the lounge when travelling on BA or QF. The flight is a BA flight. It doesn't matter that your AA frequent flyer number is on your boarding pass; just show your QC card and walk in. The AA Platinum status wil give you enhanced lounge access since you will be able to use any OW lounge when travelling on any OW carrier plus take a guest with you.

"Speaking of the QP, anyone travelling through SYD on Christmas Eve? I'm transferring from a Canberra flight to a Port Macquarie flight (arriving T3 at 1440 and departing T2 at 1540), so if anyone is at T2 that arvo, I'd like to say "G'day"."

I might be there at that time depending on what I finally figure on doing for xmas

Dave