Let's not forget that faces are not a vanity. They are a special customization feature probably made different than a vanity for lots of reasons. Would you like to think of a face like a mask that you can choose to wear one day and another like a vanity? Faces would no longer be special at all and become a regular vanity you put on and off like your everyday winged glory set (that many are getting tired of). This would dilute the entire purpose of the magic mirror. Think of the magic mirror as like a permanent and very special face paint!

Maybe suggesting a 'return feature' in which you can get the previous face you had for free, and etc. similar to your ideas.