Add a link on to /cart on /cart/checkout, using uc_cart_checkout_form. necessary or not?
The cancel button will take the user back to the cart page, which is why you are redirected there when you hit enter on the form.

You could indeed add the link by theming the checkout form.

The alternative would be by modifying the form itself, and adding a new markup type element:

$form['cart_link'] = array('#value' =&gt; l('Cart', 'cart'));


You can modify the form using hook_form_alter

But using the theme hook is fine too!