The error you are getting is in the last part of the Express Checkout response. It says:
Security Header is not valid
This means that the API Username and/or password are not correct and need to be updated. After turning off the sandbox option the error you are getting is on the checkout failure page so it seems that you might not have the correct credentials there. The headers already sent message is coming from line 1 of the checkout failure page. Make sure that you don't have any spaces outside of the php blocks on the page.
You will need to ensure that you login to your PayPal merchant account and obtain the correct values for your API username password and signature. When testing with the sandbox you will need a separate set of credentials that is specific to the sandbox. You can get these by logging into the developer area and setting up your test account.
As far as the response time goes getting several responses in a day is well within the specified response time listed at the top of the forum under hours and policy.
