When is a mirror not a mirror? Perhaps when it has been carved into and reshaped and repurposed into a well from which unreality may be drawn.
Is a doll still a doll when its emptiness is all that is put to use, filling it like a bucket from the well and drawing it back?
The doll-as-vessel is most useful when it is empty enough to carry more than just a few drops back, but not so empty that the walls become thin and it all leaks out immediately. Spilled unreality becomes a problem if it accumulates too much in one place, as it turns out.
Animated by a undifferentiated mix of entities from beyond reality, the doll-as-vessel may writhe or giggle or scream or mumble or—most usefully—speak lost secrets. Experimentation is key, as some dolls make much better vessels for this purpose than others.
Combine these Lost Secrets of Unreality with the Forbidden Knowledge one may extract from a sufficiently persuaded demon, and there is no limit to what a witch may learn. One need only take care to filter out the nonsense and lies from the true gems in her sources.