Is there any more debugging that I can do to my installation to attempt to find out what is wrong?
admin > modules > payment > PayPal IPN - Website Payments Standard > edit

At the bottom you can enable:

Debug Mode
Enable debug logging?
NOTE: This can REALLY clutter your email inbox!
Logging goes to the /includes/modules/payment/paypal/logs folder
Email goes to the store-owner address.

Then process a test order using another PP account different than the one the store uses and it will logg the details or lack there of