Well, I'd guess an ATAR of 92 isn't very realistic if you are coming "2nd last in 3U Mathematics, 10th last in English Advanced and 8th last in Chemistry" at the moment (unless there are only a dozen or so people doing those subjects in your school).
The external HSC mark is purely the HSC exam...
Bear in mind that while you *might* have been able to get a better mark for the compulsory 2-units of English for your ATAR if you had been able to do ESL rather than standard English, it probably won't affect your ATAR so much that you can't get the ATAR required to do Petroleum Engineering...
You misunderstand. Schools teach Ext 2 in Yr 12 because that's when it is due to be taught, not to be "fair". Yr 11 is filled with teaching the Math and Math Ext 1 syllabus, so it's not the time to be teaching the Yr 12 content at school.
And in some schools students CAN be accelerated and...
I think the difference is that in Physics you either know the correct equation and how to apply it, or you don't. So 0% on an answer is always a possibility in physics. And Chemistry is also somewhat like that, but it also includes more conceptual stuff where you can waffle a bit in the...