If they don't click to come back to your site, then the checkout-success page won't solve anything.
You'll have to do something in the /ipn_main_handler.php, probably just before this line:
Code:
ipn_debug_email('Breakpoint: 6 - Completed IPN order add.' . ' ordersID = '. $ordersID . ' IPN_id =' . $paypalipnID);
HOWEVER: it's important to note that if the customer makes a payment WITHOUT creating a PayPal account AND closes the browser without choosing "yes or no" to PayPal's invitation to create an account at the end of their order, the IPN is not sent, and your store will never know about the order, and thus it doesn't matter how many hooks you put in or where (you won't get emails from the store, and neither will your manufacturers). PayPal still hasn't fixed that design flaw.