The idea for adding in a different gateway for PowerStore is to essentially remove the code from the confirm page for the built in gateways. Once you have done that you can use eCart to add the local checkout that you would like to have in place.
When you started this thread you mentioned some questions that you had about setting up the local checkout server behavior, do you still have questions about any part of setting up the server behavior?
Are you going through the checkout and getting to a failure page? Please post back and let us know exactly the current issue that you are experiencing so that we can help you to proceed and get beyond it.