I've never worked with JAM so I can really give definitive answers for 1 and 2.
1) That looks correct.
2) I dont see why not
3) to ensure that the code only runs when the checkout has processed, i'd probably add the code to the webassist/plugins/shopping_cart/store_cart.php file, just before this code block:
<?php
// WA eCart Redirect
if ($WA_Store_Cart->redirStr != "") {
header("Location: ".$WA_Store_Cart->redirStr);
}
?>