@aleskupi– I got it working with the code below. A few things:
1) You had a lot of groups within groups, which is really not necessary. Groups are primarily used for advanced layouts and an add-more with an add-more.
2) As soon as I removed these parameters the Editor looked great: tabindex, editor_css and editor_class.

$addmore_icon = '  ';

//Label of meta box
piklist('field', array(
'type' => 'text'
,'field' => 'media_label'
,'label' => __('Change order by dragging items up & down.')
,'attributes' => array(
    'class' => 'hidden'
)
));

  //List of items
  piklist('field', array(
  'type' => 'group'
  ,'field' => 'media_list'
  ,'label' => 'label 1'
  ,'add_more' => true
  ,'template' => 'field' //field template for not showing label and 100% width
  ,'fields' => array(
    array(
      'type' => 'text'
      ,'field' => 'media_item_label'
      ,'label' => $addmore_icon . __('Media item')
      ,'attributes' => array(
        'class' => 'hidden'
      )
      ,'columns' => 12
    )
    ,array(
      'type' => 'file'
      ,'field' => 'media_item_background_media'
      ,'label' => __('Background image')
      ,'template' => 'custom_media'
      ,'description' => __('Select or upload box background image')
      ,'attributes' => array(
        'class' => 'piklist-custom-media-field'
      )
      ,'options' => array(
        'modal_title' => __('Background image')
        ,'button' => __('Select background image')
      )
      ,'columns' => 12
    )
    ,array(
      'type' => 'radio'
      ,'field' => 'media_item_type' // Condition for displaying fields
      ,'value' => 'video'
      ,'label' => 'Type'
      ,'description' => 'Please select what you want to show.'
      ,'choices' => array(
        'video' => 'Video'
        ,'image' => 'Image'
        )
      ,'columns' => 12
    )
    ,array(
      'type' => 'text'
      ,'field' => 'media_item_url'
      ,'label' => __('Video URL')
      ,'attributes' => array(
      'placeholder' => __('Youtube URL (http://domain.com/)')
        ,'class' => 'large-text'
      )
      ,'conditions' => array(
        array(
          'field' => 'media_list:media_item_type' // Not working. Always visible
          ,'value' => 'video' // Shown only on media_item_type = video
        )
      )
      ,'columns' => 12
    )
    ,array(
      'type' => 'file'
      ,'template' => 'custom_media'
      ,'field' => 'media_item_featured_image'
      ,'label' => __('Image')
      ,'description' => __('Select or upload media box item image')
      ,'options' => array(
        'modal_title' => __('Featured image')
        ,'button' => __('Select Featured image')
      )
      ,'conditions' => array(
        array(
          'field' => 'media_block:media_item_type' // Not working. Always visible
          ,'value' => 'image' // Shown only on media_item_type = image
        )
      )
      ,'columns' => 12
    )
    ,array(
      'type' => 'radio'
      ,'field' => 'media_item_position'
      ,'value' => 'left'
      ,'label' => 'Position'
      ,'description' => 'Select Video/Image position in box'
      ,'choices' => array(
        'left' => 'Left'
        ,'right' => 'Right'
      )
      ,'columns' => 12
    )
    ,array(
      'type' => 'text'
      ,'field' => 'media_item_title'
      ,'label' => __('Title')
      ,'attributes' => array(
        'class' => 'large-text'
        ,'placeholder' => __('Title')
      )
      ,'columns' => 12
    )
    ,array(
      'type' => 'textarea'
      ,'field' => 'media_item_subtitle'
      ,'label' => __('Subtitle')
      ,'attributes' => array(
        'class' => 'large-text'
        ,'placeholder' => __('Subtitle')
      )
      ,'columns' => 12
    )
    ,array(
      'type' => 'editor'
      ,'field' => 'media_item_description' // When add more + breaks
      ,'label' => __('Description')
      ,'attributes' => array(
        'class' => 'large-text'
      )
      ,'options' => array (
        'wpautop' => false
        ,'media_buttons' => false
        ,'teeny' => false
        ,'dfw' => false
        ,'tinymce' => true
        ,'quicktags' => true
      )
      ,'columns' => 12
    )
    ,array(
      'type' => 'file'
      ,'field' => 'media_item_quote_media'
      ,'template' => 'custom_media'
      ,'label' => __('Quote image')
      ,'description' => __('Select or upload quote logo/image')
      ,'options' => array(
        'modal_title' => __('Image')
        ,'button' => __('Select image')
      )
      ,'columns' => 12
    )
    ,array(
      'type' => 'textarea'
      ,'label' => __('Quote')
      ,'field' => 'media_item_quote_text'
      ,'attributes' => array(
        'placeholder' => __('Quote')
        ,'class' => 'large-text'
      )
      ,'columns' => 12
    )
  )
));

Let me know what you think.