My apologies for the delayed responses that you are receiving, we have an unusually high volume in support this week and have been short staffed as well. We would normally like to get the replies out quicker but we work in a queue based system and responses are made in the order posts were made.
I'm glad that you got the script to get the full request and response on the page. The error message that is the root of your troubles is this:
This transaction cannot be processed due to an invalid merchant configuration
This means that there is some agreement in your PayPal profile that needs to be agreed to or updated. Once you have this configuration issue out of the way the checkout should work just fine for you.