You seem like you have your heart set on flying, so are you going to let some guy and his aptitute test tell you otherwise? It would seem as though you've put the tail between your legs and run off in the other direction!
I don't know anything about how the ADFA works but is there anyway you can take the test again? Aside from the military way, there is of course, always the civillian option (but you said you only want to go military).
As for flight engineers, they are becoming a dying breed, well at least in civil aviation. We have just got rid of the last of our "classic" aircraft that use flight engineers - some of them were lucky enough to qualify to train as a First Officer, but most of them either retired early, or ended up with a desk job. As for the air force, I don't know how many FE's there are.
You have said that you really want to be a pilot...you don't want to be in the forces your whole life......so in my opinion, you should accept nothing less than a flying job!