Hey there, I graduated in 2018 and I did regular English, however I did English Language Units 1/2 in 2017.
I usually used the stimulus material to back up other points I was making. Depending on how many stimulus prompts there are I'd usually try to use one per paragraph, and even sometimes one in my conclusion (EL essay) to wrap everything up if it fit well.
I just quoted the stimuli and it worked out fine for me. (Please correct me if its supposed to be done another way)
And yes, you can talk about Aboriginal English and drop-in the stimulus material, I find the stimulus material works best after I had made a point and then used it to back up what I was saying. However there's nothing wrong with discussing the stimuli too and going into depth, but I know examiners will value your own quotes and examples more.
You definitely don't have to use all of the stimulus material, especially where you might just be chucking it in for the sake of it. Its more important to fit it in where it makes sense. Quality over quantity!
Hopefully this helped you out a bit

Best of luck with your studies!