Personally I'd go to Macquarie just because of ease of travel. But if you can genuinely see yourself pursuing a career in this then I suppose ANU might be the go. But don't underestimate the value of friends, family, familiarity etc.
I did extension 1 and 2 last year. Extension 1 is faaaaaar easier than English advanced and standard because there are far fewer texts (you could finish them all in a week) and the exam is 2 hours with just one essay, one creative writing. The only downside is that it is even more subjective...
There are virtually no "recent trends and statistics" you need to know. However for modern history you need to know a looooooooot of quotes, the majority of which you won't end up using in your final exam.
I did both last year. Both are very content heavy (lots to learn). However, biology is undoubtedly "easier" as there is no critical thinking. It's just memorizing facts. I think a major consideration is whether or not you're good at essays because modern history is almost purely essays whereas...
I've been told that HSC economics and legal are very dissimilar to university commerce and law respectively. Do you mean you want to do commerce/law at Macquarie or the new "stand-alone law degree." Either way I'd go for USYD. If you do well you might even be able to transfer to commerce/law.
If you're not dedicated to law, definitely go to Macquarie. If you really excel you might even be able to transfer into commerce/law at a more reputable university. Macquarie has a pretty good reputation for commerce.
You can get a 95 ATAR without any band 6's. If you got 89 in all those subjects you'd likely get over 95, but because they're low scaling try for a couple of band 6's. Your internal ranks and marks can't really be advised. Just aim for first. If you're ranked first you should be fine getting 89...