Get the renderable array for a single field instance

I'm building an interface to simplify the user experience during content creation. Users select options in three <select> fields, then on submit, they are forwarded to the correct content type add node form for the options selected. Some of the values from the original form will be be pre-filled in the new node add form.
To create this new form, I need fields from the content types and user profile. Given a content type, field name, and field instance, this is the code you need to get the Form API renderable array for the field.

/*
 * Gets the renderable array for the specified field. Call from a form function.
 *
 * @param $entity
 *   The type of data. Often 'node' or 'user'.
 * @param $bundle
 *   The entity group type. For nodes, this is the content type.
 * @param $field_name
 *   The field name of the field to render
 * @param $form
 *   The form to place the field in
 * @param $form_state
 *   The form state
 *
 */
function {site_customization_module}_get_form_field($entity, $bundle, $field_name, &$form, &$form_state) {
  // field_default_form() needs this.
  $form['#parents'] = array();
  $field = field_info_field($field_name);
  $ct_instances = field_info_instances($entity, $bundle);
  $instance = $ct_instances[$field_name];

  //See field_ui_default_value_widget() for original use.
  $items = $instance['default_value'];
  $addition = field_default_form(NULL, NULL, $field, $instance, LANGUAGE_NONE, $items, $form, $form_state);
  $form += $addition;
}