An additional ’Con’ to purchasing early is that by delaying the purchase you will have more time to ponder the best itinerary for you, become conversant in the ticket rules, ask more questions here…

Whenever you decide to purchase, it might be the case that some flights you want to book are not available – because they are seasonal; for example AA flies JKF-FCO during the northern summer and this flight might not have been loaded into the GDS when you come to book (this might mean you leave a segment free and later pay the change fee to add in the missing segment)

If looking to maximise miles, then DFW-ANC-DFW is a good earner, especially as you’d be in first class and if also PLT then you’d earn 250% miles (a total of 15,170); again this is a seasonal flight – only during the northern summer months

Most of your DONE4 flights will be in 2015
only the AMM-IST-KUL/LHR-SYD flights would be in 2014
If you maximise the flights in 2015 then you could get close to re-qualifying for PLT status on EQP (close to the 50,000 needed).
If you were able to achieve it then you’d be PLT through until 28 Feb 2017 (!)

And if you’re anything like we were when one of our daughters was living in London, you could earn truckloads of miles from your frequent visits to NYC

With your changed itinerary, have you given any more thought to how you’d position yourselves to start the DONE4 and how you’d get home at the end of it?
Are you aware that AA sells Australia-Europe return tickets that go via the US (and allow stopovers there)? The fares are reasonable (a little more than the Eastbound fares via Asia/Middle East). They require travelling on the AA codeshare/QF metal flights trans-Pacific.
And you could do the challenge this way – lots more miles.
And the return segment, which you would use after completing the DONE4 would help re-qualify for PLT in 2015