Aww, she looks like a little vulture (please don't take that the wrong way - I think she's adorable). I know nothing about this breed (we don't have them in the country I'm in) but it may just be that she needs to go through another juvenile moult before growing a tail. I just think she's so cute! Hopefully someone more knowledgeable than me will be along to admire her soon.
I'd say your right about it being a turken/americauna mix, I've seen a few americauna's with that color. The small but thick comb that extends to her nostrils is definitely an americauna trait. Unlike most pure turkens her wattles look pretty small too, another americauna trait. I've seen people call turkens with mixed colors like that a "Splash", in this case it would be a white and black splash.
I'm not sure about her tail, it will probably grow in eventually as long as shes not getting them plucked out by other chickens. Shes still adorable without one though.