1) Having it as checkout on a cart page makes more sense in general, since that's what the button is for. People are used to having checkout buttons on cart pages. Changing it the way you've specified is fine though.
2) If you plan on not having any express checkout, you can remove those files, since they are used for express checkout.
4) You would want to send the user back to just a return page. At this point you don't know if the transaction has been processed or not.
5) The user will just have to click the back button on this page.
6) You would have to set up validation using Validation toolkit or CSS Form Builder. There is client validation that you can apply to validate for phone number.
7) This could be a useful extension if you want to have a form that will send out emails to a certain group of users or to just a user that you want it sent to.