How to identify and target a form element?
You're calling the explicit hook_form_FORM_ID_alter for a start so $form_id will never be anything except question_node_form.

However the problem is you are trying to target a child of the form array. The top level array is already question_node_form. Your code should look like this:

function custom2_form_question_node_form_alter(&amp;$form, &amp;$form_state, $form_id) {
    $form['field_points']['und']['0']['value']['#default_value'] = 164;
}


Debugging removed for clarity. Noticed we removed ['question_node_form'].

Also note you can't pass $form_id by reference (not that it matters) as it can't be changed.

https://api.drupal.org/api/drupal/modules%21system%21system.api.php/function/hook_form_FORM_ID_alter/7