I’m fairly sure the same laws apply to all schemes (3 lots and upwards) with regard to the sinking fund and how those funds are managed, as well as the compulsory 10 year plan.
I know that small schemes (2 lots) can be exempt from your points above, but that certainly is not your case.
Threaten to hit the Strata Agent with the 20 penalty units under Part 4 37 (5) and see if they even know what you’re talking about 
To your other questions:
- 75A should apply to your scheme;
- The strata agent can suggest reduced numbers to the EC, but unless you have all voted to give some very strong powers to the strata agent, then they have no right to this on behalf of the OC;
- The EC represents the OC at all times. For probity’s sake, the EC/Strata Agent should document all items spent. How that documentation occurs is a matter for each scheme. Some have certain $ limits prior calling an EGM, other mundane items for day-to-day repairs can be approved via 50% of the EC on email, for instance. There is still a trail, but it won’t necessarily be minuted for you to see during each EC/AGM. Past EC members should not be involved in spending the OC money;
- The budget of the EC is the budget of the OC. This is determined by your total levies collected each year, as determined by the AGM;
- GST is nation-wide: if your levies notice says ‘Tax Invoice’ then yes, it needs to show the amount of GST, and if it has GST, then it must have the words ‘Tax Invoice’. Prospective buyers, however, are not paying levies, therefore strictly speaking they need to ask whether the amount is Inc or Ex GST.
Hope that helps