add form to /cart, commerce cart page
The way I did it was I implemented my own menu item for 'cart' and embedded the cart view on the page where I wanted it, while implementing other things I needed on the page. Definitely not for the faint of heart, though.

In this approach, your page callback is building the elements to be rendered, so you could use views_embed_view with the appropriate args if needed, pass them as variables, and render your page template where you want them.

Note: this is for Drupal Commerce in Drupal 7.