As described at that post:
... the "cart" files and "settings" files must be accessible to both the public and secure pages.
That is, during runtime, the "ustorekeeper.pl" runtime script will be accessed first via it's public URL (while the customer is browsing the store and adding items to his/her shopping cart)...and then when the customer is ready to check out, will be accessed via it's secure URL to handle collecting the customer's billing and payment information. Because the billing and payment information will be related to the items in the shopping cart, that cart file that was used save the items that the customer selected, must also be accessible to the script accessed via the secure URL.
In summary, the "ustorekeeper.pl" script that is accessed via your public (http) URL and the "ustorekeeper.pl" script that is accessed via your secure (https) URL.... must both be able to access the same physical "data" directory. Typically, this means that the public server and the secure server must be physically the same server.
So in regard to your questions:
1) Yes, you could technically run the entire store off of your secure server. This can be done by installing the scripts on your secure server and specifying all of the public URL configuration questions to be identical to the secure URL configuration questions. The obvious downside to this is that all of the regular store pages would be slower (because they would be encrypted via your secure server).
The other option is that if you use one of the built-in payment processing systems, then technically, you don't need a secure server... because the sensative payment information would be collected on the secure Authorize.Net/PlanetPayment/or LinkPoint servers....and never actually stored on your server anyway.
2) If you were to install the scripts entirely on your secure server, then yes, setup the same directory structure and everything. Just be sure that you specify all of the public URL configuration questions to be identical to the secure URL configuration questions.
3) Yes, we do have plans to develop additional payment processing interfaces.... (I think camtech is on our list for Australia).... but I can't give any estimates on when such interfaces will be complete. In the mean time, you may want to look into the PlanetPayment interface opton... As I believe "PlanetPayment" is available to merchants outsite the U.S.