jQuery not Loading for "Anonymous" Users
Drupal does not load jQuery if there are no dependency on it.

When drupal_add_js() is used once any where, Drupal includes jQuery automatically.

In case of user session, admin menu and overlay are dependent on jQuery so it is added automatically. So i advise you to try your drupal_add_js() code in hook_init just for try. I am not confirmed if it does not have any effect if it is called in hook_preprocess.

Please also check if you hook_node_preprocess is working at all?