If you've already gotten an offer for it and have just deferred, I don't see why not, but I think the best is to contact UAC or BoS themselves or go to your school or something. You're HSC is in 2016? Why are you guaranteed you'll be repeating it when you're only in prelim?
I'm pretty sure if you complete your HSC again, your new ATAR will override the old one regardless of whether you got a better score or not? correct me if im wrong.
I remember when my economics teacher had a fit at me for using a laptop when I injured my writing hand. Govt gave us laptops to use in school, I usually do pen paper always unless a situation like this, but at uni always pen + paper.