2) The checkout success page should be relied on to update the order information or send the email, for that you would need to use an IPN page.
IPN is the system that paypal uses to send the results of the transaction to your site.
the issue is this:
PayPal standard is not a real time system, once the user gets to the checkout success, that does not mean that the transaction has been processed successfully. it only means the payment method was accepted andthe transaction was placed in queu.
once the transaction is processed through the merchant bank, Paypal will send an IPN Post to a page you configure on your site. this should be used for sending the email and setting the status of the order.
Paypal Pro is a real time payment method, the checkout success page will load automatically and the transaction will have already been processed. Express Checkout is a part of payments pro.