The main justification of ur theory, the fact that the ferret is pure white can again be used against this theory.

Think about every transfiguration spell which hasn't gone particularly successfully or which doesn't have enough power behind it the product of the transfiguration retains some traits of the original object. On the top of my head I think they transfigured some teapots into tortoise or something like that and one of the tortoise had a flowery pattern on its shell.

Well what I am getting at is that this is why the ferret is pure white it is just like any other transfiguration. Malfoys white/blond hair is like his most identifying feature and it would be hard to change it.

In fact it would be almost pointless