It depends. If for say you received a non-serious attempt for your HSC exam in the course you don't want to count, they can fail your entire HSC.
But seriously....failing a course is pretty hard. Takes effort..
It's too early to tell, but I would say probably about the mid 70's range.
Ranking is in a way more important than marks, but it's no use being ranked well if your marks don't reflect it. Because you could be ranked well and still get a poor HSC mark if you don't perform well in the HSC.
I wasn't stressed. Even though I knew I wasn't going to do that well.
I somehow managed to suppress the stress and I felt fine throughout the trials and the HSC