You probably have to add the same credentials to the pp_confirm.php page. In the new system the pp_confirm.php page is never displayed and it should process the transaction and take the user directly to the checkout_success.php page. It sounds like you may still have an older version of the pp_confirm.php page and need to update it to the eCart 6 version.
The ecart doesn't currently generate code for check and bank transfer. If you want that I'd run the wizard with authorize.net as the payment gateway. Then update the confirm.php page fields to ask for bank information instead of credit card information. Then update the Authorize.net card processing code to use the bank transfer code you want to use instead. (This is something we can help with in a premier support ticket)
Where the invoice number is displayed, you can just add the "XV-" in the front. To add the zeros you can use something like:
XV-<?php echo(subtr("00000000"), strlen(strval($_SESSION['OrderID']))-8); ?>
