1. With everything else you've listed here, studying every night clearly isn't going to be an option. Rather than over-reaching yourself, set realistic goals.
It's been repeated ad nauseum on the forums but,
study smart, not hard.I'll come back to this one at the end
2. By all means attend as many parties as you like, it sounds like you've got a healthy and diligent attitude, so you should be fine. In my experience, a lot of people who have birthdays in busy SAC/ exam periods tend to schedule get togethers in quieter times so people feel less guilty about enjoying themselves at the expense of studying

Re: "girlfriend?" I realise this was only a short summary, but I hope there's more to your desire to go out with someone than the potential positive consequences for you or your grades. There are plenty of couples who make VCE work for them, but be aware that relationship drama atop all the other stresses of Year 12 can be quite damaging.
Case in point: a depressing anecdote
In the year above me, there was this guy (who we shall call Dick for reasons that shall become apparent) who broke up with his girlfriend of two and a half years on THE MORNING OF THE ENGLISH EXAM. Like, 40 minutes before it started. He'd been seeing someone else for a few weeks and she had no idea. Can't remember if her special considerations for emotional trauma got her over the line, but I do know that Dick got some awfully judgemental looks from the entire cohort that day. Especially from the teachers.
3. Surely this would be more of an in-school-hours commitment? If not, I'd imagine the school would be understanding if you needed to put this on the backburner during dense assessment weeks.
4. Sounds like you've got that sorted.
5. Again, you could probably have a chat with your parents if you ever needed a break from chores. Hobbies (esp. music/ creative ones) are great for a study break, so I'd keep those up.
6. This seems like a less flexible option (depending on where you work?) so my advice would be to stick with it during the first few weeks of Year 12 and see what the workload is like. As someone who was working 7 days all throughout VCE (averaging 40 hours between two jobs), it can definitely be done, but you need to be aware of the constraints. I was usually studying on buses/ trains/ in between meals to compensate.
Personally, I found it a nice wind-down from all the academic stuff, but given how much else you're doing, this might not be so necessary for you.
So long as you're on good terms with your employers, and they're okay with the fact that you may have to skip a couple of shifts if things get too hectic, holding a job is a pretty good idea. Especially because it'll be super hard to find work after exams, cause you'll have the whole '15 cohort who've only been worried about studying all year looking for part-time/ casual positions for the Christmas rush

Which brings me back to my first point - you don't need to sit in a library for four hours a night in order to score well. If you work better in quiet environments for long periods of time, great, but you'll have to compromise other stuff on this list in order to achieve this.
However, (and I'm assuming this is the case,) there are probably more effective ways for you to learn and revise throughout the year. Can't speak for any of the science subjects, but doing well in Englang and Art (theory component) usually require a solid conceptual understanding developed throughout the year, rather than 'spamming' practice exams for hours on end.
Plus, using class time efficiently is an often ignored way to do better in subjects. Yes, most will require study outside of class hours, but if you're actually working with your teachers and peers to get the best out of your time physically at school, you can cut down on study hours considerably.
Rather than coming to a decision right now, perhaps see how you go during February. Since you'll be able to plan out SACs well in advance, hopefully you'll be able to achieve a good balance between everything on this list.
I suppose rather than seeing it as 'sustaining all these things for the duration of year 12,' take it day by day, week by week, and see how you feel
