the mirror's surface is an infinitely thin boundary between the real and unreal, where the unreal can touch the real provided it doesn't causally act on it. this permits magic because it's possible for two seperate reasons for something to be true at the same time - there can be many paths leading to the present, of which any number can be true as long as only one of them is causally true. the trick is, it doesn't actually matter which one that is, the magical explanation is just as meaningfully true now that the past is inaccessible. magic cannot act causally because of the violet nature of reality (though if it could then it would also render all choice meaningless because of the mistake).
behind the mirror's surface is a creature of eyes and wings and tentacles, and where it presses against the glass and infinitely gently touches the world i am, in some sense, its appendage.
it is possible that the mirror is a door. it's a matter of perspective, i think.