1.) Please investigate. According to 2Checkout, Marketpress sends the wrong information to their system, which is why is adds extras pages - such as the customers address. It is not necessary to collect that a second time.
2.) I just tested in sandbox mode today - now I get error PE102. According to helpfile, it seems this is most likely caused by using the wrong URL to pass info in sandbox mode. It should go to: https://sandbox.2checkout.com/checkout/purchase