1) you can update the hidden form elements on the bottom of the page to alter what is sent to paypal and include an invoice number. The email may still be referring to the old session variables. You should just update so all of the shipping session variables are set from the billing form.
2) You would have to check the paypal api, but I think you may just need to delete the tax hidden form element on the confirm page and that will cause it to use the paypal tax calculation.