Always adopt the mentality of aiming for the best possible outcome.
But of course, don't get too anxious- put things into perspective i.e. it's worth 5%
I found Economics interesting, useful and enjoyable at the same time. It's applicable to so many things- even aviation-cost of wages for pilot, cost/benefit analysis of particular routes, etc, etc.
Anyway, Physics is of course useful too.
Ultimately it's up to you
Don't work yourself excessively, but at the same time try to do the best you can. You want to get into the mindset that you are aiming for the top, so then you will panic less in yr11 and 12.
Yeah, enjoy yr 10.
Number of hours depends: on the individual, on how much time you feel like setting aside, etc, etc.
I guess make sure you understand theory/concepts, and ask for help on any topic you have difficulty with.
Just remember to study consistently.
Make sure you know all the theory concepts first, making sure to ask teachers/tutors for help if there are any holes in your understanding.
Then do practice papers- your school's past exams for your internal assessments, and perhaps actual HSC papers if you have time.
Cadetships and internships are similarish- I guess cadetships tend to be for the duration of the cadet's degree, while internship/clerkships tend to be only partly i.e a few weeks/few months.
Volunteering is just that- you don't get paid.