Default for a content type

Thanks for the encouragement! :o)

I've had a go at putting something together (my first attempt at php of any significance!). I used your module and view_mode_modal as guides and came up with my attempt below. It works for me but it obviously needs a proper review and testing. It's also untidy so far. For example, if you delete a context or view mode, I don't clean up what's stored for the content type. It doesn't seem to cause problems (Drupal seems to handle the case with reasonable defaults) but ideally it would be better tidied up.

Anyway, comments welcome!

/**
 *  Hook to set up the context view mode options for each of the possible display modes.
 *  The user chooses a context for each of the view modes if wanted.
 *
 */

function mymodule_form_field_ui_display_overview_form_alter(&$form, $form_state, $form_id) {

  $entity_type = $form['#entity_type'];
  $bundle = $form['#bundle'];

  $bundle_settings = field_bundle_settings ($entity_type, $bundle);
  //load entity information
  $entity = entity_get_info($entity_type);

  // Load current settings if any
  $context_view_modes_saved = isset($bundle_settings['context_view_modes']) ? $bundle_settings['context_view_modes'] : '';
  
  // Add additional settings vertical tab if not already there.
  if (!isset($form['additional_settings'])) {
    $form['additional_settings'] = array(
      '#type' => 'vertical_tabs',
      '#theme_wrappers' => array('vertical_tabs'),
      '#prefix' => '<div>',
      '#suffix' => '</div>',
      '#tree' => TRUE,
    );
    $form['#attached']['js'][] = 'misc/form.js';
    $form['#attached']['js'][] = 'misc/collapse.js';
  }
  
  $form['additional_settings']['context_view_modes'] = array(
    '#type' => 'fieldset',
    '#title' => t('Contextual display mode settings'),
    '#description' => t('You can link view modes to specific contexts but be aware they will be overridden if the nodes have their own settings.'),
    '#collapsible' => TRUE,
    '#collapsed' => TRUE,
    '#parents' => array('additional_settings'),
    '#weight' => 100,    
  );

  $contexts = array('default' => '-Default-');
  $contexts += context_context_list();

  $ds_modes = ds_entity_view_modes('node');

  foreach($ds_modes as $key => $mode) {
    $form['additional_settings']['context_view_modes'][$key] = array(
      '#type' => 'select',
      '#title' => t("Use ") . $mode['label'] . t(' when context:'),
      '#options' => $contexts,
      '#default_value' => isset($context_view_modes_saved[$key]) ? $context_view_modes_saved[$key] : '',
    );
  }
  
  //add a custom submit handler
  $form['#submit'][] = 'mymodule_field_ui_display_overview_form_submit';
  
}


/**
 * Submit handler for field_ui_display_overview_form form
 */
function mymodule_field_ui_display_overview_form_submit($form, &$form_state) {

  $form_values = $form_state['values'];
  $entity_type = $form['#entity_type'];
  $bundle = $form['#bundle'];
  $view_mode = $form['#view_mode'];

/*
*  We need to know all the possible view modes so we can loop over them and read what the user has set for them
*/
  $ds_modes = ds_entity_view_modes('node');

  $context_view_modes_new = array ();
  foreach($ds_modes as $key => $mode) {
    $context_view_modes_new[$key] = $form_values['additional_settings'][$key];
  }
  
  // Get current bundle settings and update them with the new context view modes (if any)
  $bundle_settings = field_bundle_settings($entity_type, $bundle);
  $bundle_settings['context_view_modes'] = $context_view_modes_new;

  // Save updated bundle settings.
  field_bundle_settings($entity_type, $bundle, $bundle_settings);

}

/**
 *  Now choose the mode to use to display the node
 * @param  $build  The build array that drupal_render() expects
 */
function mymodule_node_view_alter(&$build) {

 // To avoid an endless loop do this v
  static $call_once;
  $call_once++;
  if($call_once >= 2) { return; }

  // Only work with nodes when on page view
  $a0 = arg(0);
  $a1 = arg(1);
  if($a0 !== "node" || !is_numeric($a1)) { return; }

  // Get current node
  $node = $build['#node'];

  // If v is not set then we want to run our version of ds_extras display switch
  if(isset($_GET['v'])) { return; } // v is set. Dont eff with that

  if(!isset($node->cvw)) {  // **** I to change this from your version as it didn't seem to work for me ****

    //Get bundle and entity type then get the settings that include the contextual display modes for the content type (if any)
    $bundle = $build['#bundle'];
    $entity_type = $build['#entity_type'];
    $bundle_settings = field_bundle_settings($entity_type, $bundle);

    if (!isset($bundle_settings['context_view_modes'])) { return;}  // Nothing to do!
    else {
  	 
    	 /*
      *  We need to know all the possible view modes so we can loop over them and match them with the active context
      *  (if more that one use the first)!
      */
      $ds_modes = ds_entity_view_modes('node');
      $contexts = context_active_contexts();

      $context_view_modes_new = array ();
      foreach($contexts as $context_name => $context) {
        foreach($ds_modes as $key => $mode) {
          if ($bundle_settings['context_view_modes'][$key] == $context_name) {
            $build = node_view($node, $key);
            return; // take only the first match
            }  
          }
        }
  	   }
  	 }

  // The content type has no context display modes so check if the node itself has  	 
  	 
  // Get all of the contexts that have a match
  $contexts = context_active_contexts();

  // Loop through each context and see if this node has a view mode assigned
  // to a valid context
  foreach($contexts as $context_name => $context) {
   if(isset($node->cvw[$context_name]) && count($node->cvw[$context_name]) && $node->cvw[$context_name] !== "default") {
      $node->ds_switch = $node->cvw[$context_name];
      $build = node_view($node, $node->ds_switch);
      return; // take only the first match
    }
  }

}

As I said, I used view_mode_modal as a guide as well as your original. I'll be happy with whatever you decide. Just need to be sure that there's a hierarchy of settings so that if you set it up for, say, an individual article, it "trumps" the setting for articles in general.

I'm using my approach at the moment but I'll move over to yours when you release it.

Fun, fun, fun!