Charn is only talked about in MN and how Jadis gets to Narnia is only talked about in MN. Like it has been stated in almost every thread, Lewis didn't intend on making the series longer, so he did leave loose ends. But Since her sister was the rightful heir to the thrown, it would probably have been Jadis' and her heirs that were supposed to have been there, but since Jadis uttered the deplorable word-----it changed everything.