I agree that it looks like a runner in the last picture. Did you get it at a feed store? If so do you know what hatchery they got the ducklings from? If it was from a feed store it's not likely to be a mix breed.
Like I said before it could be an Ancona in lilac, silver, or one of those type colors but I don't know if any hatcheries sell those verities. And I don't know enough about the genetics to know if color variations can just show up from two black and white Anconas pairing.
You really should check the protein percentage on your feed. I'm pretty sure gamebird feed is like 24-30% protein and that is WAY too high. Higher protein causes them to grow faster then they should which can cause bone structure problems.