Dynamic form elements and node edit form.

Hi. I created a module with a node type (let’s call it "game_tip"). This node type has to be with 2 fields (in addition to default "title" and "body"). The first is the "game" and the second is the "trophy". I defined them into mymodule_form() as follows:

  $form['tip_options']['game'] = array (
    '#type' => 'select',
    '#title' => t('Game'),
    '#weight' => -4,
    '#options' => (array)mm_get_gamelist(),
    '#suffix' => "<div class='clear'></div>",
    '#ahah' => array(
      'path' => 'mymodule/js/trophylist',
      'wrapper' => 'edit-trophy-wrapper',
      'method' => 'replace',
      'event' => 'change',
    ),
  );


  $list = mm_get_trophylist((int)$form_state['values']['game']);
  $form['tip_options']['trophy'] = array (
    '#type' => 'select',
    '#title' => t('Game trophy'),
    '#weight' => -3,
    '#disabled' => $list == FALSE,
    '#options' =>$list ? $list : array(0 => t('Select game first')),
  );

As you can see these two fields are required. Just like "body" and "title" fields by the way =)
As written here and here I created my own callback function:

function mymodule_trophylist_js() {
  include_once 'modules/node/node.pages.inc';

  $form_build_id = $_POST['form_build_id'];

  $form = form_get_cache($form_build_id, $form_state);

  $args = $form['#parameters'];
  $form_id = array_shift($args);
  $form_state['post'] = $form['#post'] = $_POST;
  $form['#programmed'] = $form['#redirect'] = FALSE;

  $form_state = array('storage' => NULL, 'rebuild' => TRUE, "force_validate" => TRUE);
  drupal_process_form($form_id, $form, $form_state);
  $form = drupal_rebuild_form($form_id, $form_state, $args, $form_build_id);

  $choice_form = $form['tip_options']['trophy'];
  unset($choice_form['#prefix'], $choice_form['#suffix']);
  $output = theme('status_messages') . drupal_render($choice_form);

  drupal_json(array('status' => TRUE, 'data' => $output));
}

The idea of what I need is when I select the game, the callback function shall return a trophy list for this game. Everything works fine, but when I select the game the callback function does not work properly it returns me an error message with the following text:

Title field must have a value.
Body field must have a value.

and all other error messages that mymodule_validate() function generate.
Can I use AHAH there? And if 'yes' where am I wrong?