Easy, easy, easy, may be too much, but at least easier.
Your change should look something like this:
<?php
class Custom_Integrate
{
	public static function register()
	{
		// $hook, $function, $file
		return array(
			array(
				'integrate_menu_buttons',
				'Custom_Integrate::menu_buttons',
			),
		);
	}
	public static function menu_buttons(&$buttons)
	{
		global $modSettings, $scripturl;
		$buttons = elk_array_insert($buttons, 'pm', array(
				// Eigener Button zu den Entwürfen.
				'drafts1' => array(
					'title' => 'Entwürfe',
					'href' => $scripturl . '?action=profile;area=showdrafts',
					'show' => !empty($modSettings['drafts_enabled']) && !empty($modSettings['drafts_post_enabled']),
				)
			),
			'after',
			false
		);
	}
}
And you would be able to enable/disable the "integration" directly from the admin panel.