Add to cart OR show file download

Half a year later I stumbled across the same issue and somehow managed to make my own formatter that works the way I want:
Show a Download link for users who have bought a file or have permission to bypass and show a add-to-cart form for everybody else.

For future reference here's the code:

https://gist.github.com/klickreflex/f917479dca40211a7624962cf76efaa8

/** 
 * @file
 * adds a formatter for commerce file fields that shows an add to cart button OR 
 * a file download link, depending on whether the current user has a license for the file
 *
 */
/**
 * Implements hook_field_formatter_info().
 */
function si_formatters_field_formatter_info() {
  return array(
    'add_to_cart_or_download_formatter' => array( // machine name of the formatter
      'label' => t('Add to Cart or Download'),
      'field types' => array('commerce_product_reference'), // this will only be available for product reference fields
      'settings' => array (
        // custom setting for the Download link
        'download_text' => t('Download now'), //ditto
        // Settings from the original add to cart form:
        'show_quantity' => FALSE,
        'default_quantity' => 1,
        'combine' => TRUE,
        'show_single_product_attributes' => FALSE,
        'line_item_type' => 'product',
      ), // Array of the settings we'll create
    ),
  );
}
/**
 * Implements hook_field_formatter_settings_form().
 */
function si_formatters_field_formatter_settings_form($field, $instance, $view_mode, $form, &$form_state) {
  //This gets the view_mode where our settings are stored
  $display = $instance['display'][$view_mode];
  //This gets the actual settings
  $settings = $display['settings'];
  //Initialize the element variable
  $element = array();
  $element['download_text'] = array(
    '#type'           => 'textfield',                        // Use a textbox
    '#title'          => t('Download Now'),                      // Widget label
    '#description'    => t('The download link text for users who are allowed to download without checkout.'),  // helper text
    '#default_value'  => $settings['download_text'],               // Get the value if it's already been set
  );  
  $element['show_quantity'] = array(
    '#type' => 'checkbox',
    '#title' => t('Display a textfield quantity widget on the add to cart form.'),
    '#default_value' => $settings['show_quantity'],
  );
  $element['default_quantity'] = array(
    '#type' => 'textfield',
    '#title' => t('Default quantity'),
    '#default_value' => $settings['default_quantity'] <= 0 ? 1 : $settings['default_quantity'],
    '#element_validate' => array('commerce_cart_field_formatter_settings_form_quantity_validate'),
    '#size' => 16,
  );
  $element['combine'] = array(
    '#type' => 'checkbox',
    '#title' => t('Attempt to combine like products on the same line item in the cart.'),
    '#description' => t('The line item type, referenced product, and data from fields exposed on the Add to Cart form must all match to combine.'),
    '#default_value' => $settings['combine'],
  );
  $element['show_single_product_attributes'] = array(
    '#type' => 'checkbox',
    '#title' => t('Show attribute widgets even if the Add to Cart form only represents one product.'),
    '#description' => t('If enabled, attribute widgets will be shown on the form with the only available options selected.'),
    '#default_value' => $settings['show_single_product_attributes'],
  );
  return $element;
}
/**
 * Implements hook_field_formatter_settings_summary().
 */
function si_formatters_field_formatter_settings_summary($field, $instance, $view_mode) {
  $display = $instance['display'][$view_mode];
  $settings = $display['settings'];
  $summary = t('Use either Add to Cart button or a Download link (depending on file license)'); 
  return $summary;
}
/**
 * Implements hook_field_formatter_view().
 */
function si_formatters_field_formatter_view($entity_type, $entity, $field, $instance, $langcode, $items, $display) {
  if ($display['type'] == 'add_to_cart_or_download_formatter') {
    $result = array();
  
    $settings       = $display['settings']; // get the settings
    $download_text  = $settings['download_text']; // The label assigned in settings
    // Collect the list of product IDs.
    $product_ids = array();
    foreach ($items as $delta => $item) {
      $product_ids[$item['product_id']] = $item['product_id'];
    }
    // Exit now if we didn't find any product IDs.
    if (empty($product_ids)) {
      return;
    }
    // Load the referenced products.
    $products = commerce_product_load_multiple($product_ids, array('status' => 1));
  
    foreach ($products as $delta => $product) {
      // Protect ourselves from recursive rendering.
      static $depth = 0;
      $depth++;
      if ($depth > 20) {
        throw new CommerceProductReferenceRecursiveRenderingException(t('Recursive rendering detected when rendering product (@product_id). Aborting rendering.', array('@product_id' => $item['product_id'])));
      }
      $fid = $product->commerce_file['und'][0]['fid'];
      $file = file_load($fid);
      // the file description is used for the download link, 
      // so we set/override it here with the display setting
      $file->description = $download_text;
      $download_link = theme('file_link', array( 'file' => $file ));
      $element[0]['#markup'] = $download_link;
    }
    global $has_license;
    $has_license = false;
    if(commerce_file_access('download', $file)) {
      $has_license = true;
      return $element;
    }
  
    else {
      // show add to cart button if user has no license
      $settings = array_merge(field_info_formatter_settings($display['type']), $display['settings']);
      // Collect the list of product IDs.
      $product_ids = array();
      foreach ($items as $delta => $item) {
        if (isset($item['product_id'])) {
          $product_ids[] = $item['product_id'];
        }
        elseif (module_exists('entityreference') && isset($item['target_id'])) {
          $product_ids[] = $item['target_id'];
        }
      }
      // Load the referenced products.
      $products = commerce_product_load_multiple($product_ids);
      
      // Check to ensure products are referenced, before returning results.
      if (!empty($products)) {
        $type = !empty($settings['line_item_type']) ? $settings['line_item_type'] : 'product';
        $line_item = commerce_product_line_item_new(commerce_product_reference_default_product($products), $settings['default_quantity'], 0, array(), $type);
        $line_item->data['context']['product_ids'] = array_keys($products);
        $line_item->data['context']['add_to_cart_combine'] = !empty($settings['combine']);
        $line_item->data['context']['show_single_product_attributes'] = !empty($settings['show_single_product_attributes']);
        $result[] = array(
          '#arguments' => array(
            'form_id' => commerce_cart_add_to_cart_form_id($product_ids),
            'line_item' => $line_item,
            'show_quantity' => $settings['show_quantity'],
          ),
        );
      }
    return $result;
    }
  } // if add_to_cart_formatter
}
/**
 * Implements hook_field_attach_view_alter().
 *
 * When a field is formatted for display, the display formatter does not know
 * what view mode it is being displayed for. Unfortunately, the Add to Cart form
 * display formatter needs this information when displaying product reference
 * fields on nodes to provide adequate context for product field replacement on
 * multi-value product reference fields. This hook is used to transform a set of
 * arguments into a form using the arguments and the extra context information
 * gleaned from the parameters passed into this function.
 */
function si_formatters_field_attach_view_alter(&$output, $context) {
  
    // Loop through the fields passed in looking for any product reference fields
    // formatted with the Add to Cart form display formatter.
    foreach ($output as $field_name => $element) {
      if (!empty($element['#formatter']) && $element['#formatter'] == 'add_to_cart_or_download_formatter') {
        global $has_license;
        if(!$has_license) {
          // Prepare the context information needed by the cart form.
          $cart_context = $context;
          // Remove the full entity from the context array and put the ID in instead.
          list($entity_id, $vid, $bundle) = entity_extract_ids($context['entity_type'], $context['entity']);
          $cart_context['entity_id'] = $entity_id;
          unset($cart_context['entity']);
          // Remove any Views data added to the context by views_handler_field_field.
          // It unnecessarily increases the size of rows in the cache_form table for
          // Add to Cart form state data.
          if (!empty($cart_context['display']) && is_array($cart_context['display'])) {
            unset($cart_context['display']['views_view']);
            unset($cart_context['display']['views_field']);
            unset($cart_context['display']['views_row_id']);
          }
          // Add the context for displaying product fields in the context of an entity
          // that references the product by looking at the entity this product
          // reference field is attached to.
          $cart_context['class_prefix'] = $context['entity_type'] . '-' . $entity_id;
          $cart_context['view_mode'] = $context['entity_type'] . '_' . $element['#view_mode'];
          $entity_uri = entity_uri($context['entity_type'], $element['#object']);
          foreach (element_children($element) as $key) {
            // Extract the drupal_get_form() arguments array from the element.
            $arguments = $element[$key]['#arguments'];
            // Add the display path and referencing entity data to the line item.
            if (!empty($entity_uri['path'])) {
              $arguments['line_item']->data['context']['display_path'] = $entity_uri['path'];
            }
            $arguments['line_item']->data['context']['entity'] = array(
              'entity_type' => $context['entity_type'],
              'entity_id' => $entity_id,
              'product_reference_field_name' => $field_name,
            );
            // Update the product_ids variable to point to the entity data if we're
            // referencing multiple products.
            if (count($arguments['line_item']->data['context']['product_ids']) > 1) {
              $arguments['line_item']->data['context']['product_ids'] = 'entity';
            }
            // Replace the array containing the arguments with the return value of
            // drupal_get_form(). It will be rendered when the rest of the object is
            // rendered for display.
            $output[$field_name][$key] = drupal_get_form($arguments['form_id'], $arguments['line_item'], $arguments['show_quantity'], $cart_context);
          }
        }
      }
    }
  //} // end check
}