Hey,
It sounds like you've had an incredibly demanding year. I would strongly encourage you to fill out a SEAS application and put in the required evidence. It can be draining to go back through and read/write about what's been going on, but it's a great first step to make sure that unis can make a more informed choice about selecting you.
If your school has access to edrolo, I'd recommend making a plan to get through watching all of the videos by 5 days before the exam. If not, you may be able to find other relevant videos for your subjects.
Practice questions are also your friends. Try not to be too fazed if you get many of them wrong at the start - this is to be expected.
You might be tempted to skip out on sleep to aid your catching up. If only do this if you have a test within the next couple of days and you don't know the content. Otherwise, try to protect your sleep as it will aid your learning and wellbeing.
I don't know you well enough to have any kind of guess about whether 90+ is realistic for you. It could be.
I'd also like to remind you that your ATAR does not determine your future job, it does not determine your quality of life, it does not determine your worth. It's a useful key, but there are always other paths onto the room. Once you get into a course no one cares how you got there.
Some day in the future these difficult times will be reflected on as resilience and empathy building adversity, and although no one should be put through these trials, you'll no longer feel trapped by them.
I believe in you, and future-you believes in you too