doing 10 units is good. it isnt worth it to study a whole syllabus plus exams and assignments just for insurance. anyway, if you take 10 units only, then you have a lower work load, meaning you can keep going ahead and revising, which means you probably wont need an extra subject. im planning on...
did you pick the subjects that you thought would give you the best atar possible, or did you pick the subjects that you thought looked good? you dont need to feel "inadequate" because of the subjects you chose. there are many people who have earned themselves a high atar with "low-scaling subjects".
It is possible but extremely unlikely. You always have a chance, but with subjects like standard english and bio, you're going to have to ace everything.