We did the Darwin-Cocos part a couple of years ago (and longer ago than that). Seems like you have it covered, the only trick is knowing where/when to find wind
getting out of Darwin, just a matter of watching the forecast
We found a stop at Ashmore Reef well worth the minor paperwork with the ADF.
For the return part, that question got asked on Noonsite
a while back, specific to September, and got answered by none other than Jimmy Cornell himself. The opening line of his response:
Out at Cocos the SE winds stick around pretty much all year. Where on the mainland do you want to end up? If back at Darwin I would start to think about routes that take you north to Indo, then east through the archipelago, then back south. If West Coast
then maybe waiting for a high to pass south of Cocos to allow the wind
to come a bit more east and see how much you can make of it.