eCart can be added to a PHP File.
It can integrated using jquery lightbox elements, but that will take custom coding to accomplish.
To display the cart on a mobile device, you will likely need to edit the display manager css.
I will put through a feature request for mobile friendly designs in eCart.
We do not have plans on integrating paypal adaptive payments.
The eCart beta has not started yet. we have received your application, but have yet to get to the testing stage. Once we get there, we will send out emails to the beta participants.