Simon, did your wife specifically tell the salespeople at the store that you would want a scope for DSO imaging, i.e. an EQ mount was clearly warranted? If so, the goods that were sold were clearly not fit for purpose, and you're entitled to a remedy (exchange or refund being most likely here):
http://www.accc.gov.au/business/trea...ts-obligations
However, as you point out - some businesses can be a bit dodgy in following the law. Purchasing via credit card gives you another avenue to address the situation, but the consumer law still applies regardless of how your wife paid.
If the business is particularly annoying about it, your only real remedy to is file a legal claim with your state's small claims tribunal. It's cheap and efficient (no lawyers involved), but may not be worth the opportunity cost of your time.
Perhaps you could use this to negotiate a positive outcome to the situation? For example, maybe negotiate for a reasonable discount on your Option C?