to pass the billing details to paypal, edit the confirm page to use the billing sessions instead of the shipping sessions in the hidden elements that pass to paypal, for example::
change:
<input type="hidden" name="first_name" id="first_name" value="<?php echo((isset($_POST["shipping_firstname"]))?$_POST["shipping_firstname"]:"") ?>" />
to:
<input type="hidden" name="first_name" id="first_name" value="<?php echo((isset($_POST["firstname"]))?$_POST["firstname"]:"") ?>" />