After catching up on the thread, there seems to be two open items that it would be VERY useful for RedRoo to confirm.
1. If you register and book a flight in the qualifying period, but later change the date (but still fly before the flight deadline), the flight will or will not earn double SC.
T&C seem to address this situation.
Full or part travel of an itinerary is eligible only if you book and travel within the promotional dates as listed above. If you change or cancel your initial qualifying booking and/or travel is completed outside the promotional period, this bonus offer will not apply
So I read 2 things - as long as flights are between April 28 and April 16, 2017 they qualify
if you somehow had a booking (or changed it) to say leave on 15 April 2017 and return on 18 April 2017, the outbound sector(s) would count, but not the inbound.
And-
if you have a booking made during the eligible period (ie: April 28-May 4, 2016) for flights between April 28, 2016 and April 16, 2017 but then change it (eg: extend the return beyond April 16) tha changed sector won't qualify, but if you change it and flights are still within the period (eg: book flights for Oct 1-10 this year, and change to Nov 1-10 then they would still qualify)
I think the wording is reasonably clear to me on this.
As it is booking now you can't book past APril 16th due to the QF Amadeus ticketing limit anyway so sure you could book flights for say early April next year, then change them, but the wording still suggests any sectors flown between now and April 16, 2017 will get DSC
further if you cancel a booking on say May 8, even if you rebook same flights obviously you won't get any DSC, and any residual voucher issued wouldn't count either.