1) Which one did you like better, or enjoyed studying more in VCE? That might help you decide which subject you would rather take. No point taking physics, etc, if you hate it--you just won't be motivated to study for it...and so there goes your high GPA.
2) i have no clue about GAMSAT with physics. Can't help you there. But again, just consider your capabilities. Did you struggle terribly trying to get your study score for physics?
3) If you were to do Calc 1 in Sem1...and enjoy it and possibly see yourself continuing, take calc 2. data analysis, i dunno, sounds kind of boring. it's more quantitative stuff, not at all like that "pure" maths stuff you would find if you did calculus or something like that.
But, with data analysis, i think it is apparently useful for some majors. i don't remember which one it was, (pysch? not relevant, but oh well, just putting it out there) but a prereq for that major was something quantitative--so if you did data analysis, it might help. Alternatively, just look at the handbook..and do a search of other majors that you haven't considered--perhaps you can decide on your subjects that way.
4) No clue!