How can I hide node titles only for displayed nodes, and not node edit forms?
You could add a check to see if you are editing the node, like

function mytheme_alpha_preprocess_page(&amp;$vars) {
  if (isset($vars['node']) &amp;&amp; $vars['node']-&gt;type == 'episode') {
    if(arg(2) != 'edit') {
        $vars['title_hidden'] = TRUE;
    }
  }
}