Inline Entity Form - How to change label for the EDIT button and REMOVE button
Try form after build. it may give you solution.

function MY_MODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id == 'CONTENTTYPE_node_form') {
    /*
    * Modify the Add node button when it's the first one
    */
    $form['MY_FIELD']['und']['form']['actions']['ief_add_save']['#value'] = t('CUSTOM TEXT');
    /*
    * Modify the Add node button when there is at least one node
    */
    $form['MY_FIELD']['und']['actions']['ief_add']['#value'] = t('CUSTOM TEXT');
    $form['#after_build'][] = 'yourmodulename_after_build'; //Calls function 
  }
}


Use dpm($form) or print_r($form) to find the edit button id