This is all kind of over my head here and possibly off topic, but to aid in anonymity in any mixer no matter the number of inputs/outputs and no matter who inputs the coins, what about the possibility of instead of immediately crediting the outputs, the mixer could issue a "receipt" that could be cryptographically printed on a smartchip or something. This "receipt" can be used just like physical cash and passed around in transactions with no record in the blockchain until it is redeemed to the mixer later, which adds anonymity and the convenience of physical cash.
It was mentioned above that you indeed can get better anonymity if you trust the mixer... That's just plain Chaumian cash.
