It sounds wonderful and a great itinerary, packing a lot in to get more of a feel for the place than just dropping in at ports.
BTW I loved the aquarium in Cairns, was there last January on a HAL cruise. Darwin is hot, I've been there several times, but the heat and humidity does not bother me as it does some. Over 30C/86F almost every day of the year.
Thanks for the update.