How do I disable the cache clear on comment form submission?
Should be reasonably straight forward with a form alter (mileage may vary slightly here but this is the general idea):

function MYMODULE_form_comment_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  $key = array_search('comment_form_submit', $form['#submit']);
  if ($key !== FALSE) {
    $form['#submit'][$key] = 'MYMODULE_comment_form_submit';
  }
}

function MYMODULE_comment_form_submit($form, &amp;$form_state) {
  // Copy code from comment_form() here, remove what you don't need.
}