you really shouldnt be going so far ahead. half a topic should be fine. otherwise, you'll end up revising previous stuff again and studying stuff that's not even in your half-yearly exams.
not all schools finish all the topics so don't worry.
Remember, Preliminary material becomes ASSUMED KNOWLEDGE for the HSC and may be still examinable. so dont forget to study those prelim topics.
you can't add subjects for the hsc in year 12. you can only drop subjects. it'd probably be better if you dropped from 14 units to 12. 14 is way too much work.