The problem is the trigger you are using to populate the passed cart is set to "any form post", which causes it to try to reset the cart contents when you click the express checkout button.
The solution is to trigger from the original button name, so instead of:
// WA eCart Get Form
if ($_SERVER["REQUEST_METHOD"] == "POST") {
use:
// WA eCart Get Form
if (isset($_POST['Checkout'])) {