drupal commerce - when adding to cart on review page, get redirected back to checkout
I had this exact same problem with a very similar workflow.  I created a workaround to this issue by integrating the commerce_cart_product_add($uid, $line_item) function from the Commerce Cart module into my custom module.  This way I could test what was happening in Commerce without hacking the contrib module.  From there I discovered that the issue stemmed from an invoked rule at the bottom of this function:

 rules_invoke_all('commerce_cart_product_add', $order, $product, $quantity, $line_item);


Removing this rule from my local version of this function solved the redirect issue. I looked through the Commerce Settings and the Rules UI in the Drupal admin to see if I could find this rule to disable it, but I could not find a reference to it anywhere.  This is not the best solution, but for now it solves the redirect issue.  I would be interested to hear if anyone else has run into a similar issue though.