in this case, the headers already sent error is happening because of the first error.
the first error is the header that has already been sent.
so to fix it, you will need to fix the first error. if you need help with that, send a copy of the cart object and the eCart_definition files in the WA_eCart folder.
as for the pages that are created, the pp.... pages that you mention are created only if you select to use PayPal Express checkout as the gateway. if you use a different gateway, and deselect the option to include Express checkout, the pp... pages are not needed, so not created.