How do I redirect back to the source page after the user submits a comment?
To redirect a form after submission you would do this:

/**
 * Implements hook_form_FORM_ID_alter().
 */
function MYMODULE_form_comment_form_alter(&amp;$form, &amp;$form_state) {
  // Add a custom submit handler to act when the form submits.
  $form['#submit'][] = 'MYMODULE_custom_comment_submit_handler';
}

/**
 * Custom submit handler for the comment form.
 */
function MYMODULE_custom_comment_submit_handler($form, &amp;$form_state) {
  // Redirect the user after submission
  $nid = arg(2);
  $form_state['redirect'] = 'mymodule/' . $nid;
}