Well a good thing to keep at the back of your mind is that you always have the opportunity to repeat Year 12, while it may not be the most desirable path to take, it's an option which is there and if, by whatever means, you don't get the ATAR you think you're capable of, there's always another chance for you to have a fresh start and just do it all again.
I know that it might not be the best option especially given that you're nearly done, but you know, it does have its benefits, you'll be able to start over again with a fresh mind, carrying nothing over from this year and you won't have any qualms, regrets or "what ifs" about how you go and that's always something which is very comforting. I'd never be able to live with myself if I knew something affected my VCE and I could have done better if I had another chance (but I can be competitive like that).
In terms of applying for a derived examination score, I personally know someone who applied for one and he was able to do it through the school's Head of Senior School. So essentially he just had to get all the required medical paperwork which was outlined by VCAA and bring it to school and they were able to do it all for him. Always remember that school staff are all there to support you and they probably have come across many similar cases to yours before, so they will have experience about what to do, so make sure you make use of their experience and knowledge.
Anyhow, I wish you all the best for your exams, by showing such conviction to wanting to do well on VCE, you would have no doubt made your parents and more importantly your own self very proud. Like I often say, it's not only the end result that matters, but it's the experiences which you gain and deal with on the way which truly shape who you are. Good luck.