Hi Brendon and welcome to the forum,
In regards to using your current account to sell into the US, that's really a personal call for you to make, there is certainly nothing preventing you doing that. As you have correctly identified, although you are located in Australia, the location description has to reflect where the item actually is, not where the seller is located.
Even if you started another account, that wouldn't change your account information, buyers would still be able to see you were an Australian member.
I would suggest along with ensuring the item location is correct in your listing, also heavily emphasize the fact that the item is being sent direct from the USA. That slong with highlighting quick shipping to US customers will hold the buyer's attention on the product itself and they won't be the least bit concerned where you as the seller is located.
Regarding settings, if you are selling through the one account then that's really out of your control. All you can do is list your US focused sales on US eBay and your Australian ones on eBay Oz, then it comes down to the buyers personal serach settings as to what they can see. If they select a worldwide search, then if you have products that fall under the search terms the user selects, they'll all display, no matter what country you are listed in.
If you want to keep them completely separate, your only option would be to use two different accounts. Of course that wouldn't prevent the products from both accounts still showing (as aboce scenaro), but they will show under different seller's names which at least gives you some basic level of anonymity, if that is what you require.