Yes, there definitely is a resemblance to Cumberbatch. Picturing this dragon face with Benedict's voice and presence, I have little to no doubt in my mind that the Smaug scene with Bilbo will be amazing. So far everything seems as close to perfect as possible.
I don't think we can, his head's back could be of any shape
I just watched AUJ two days ago and did a freeze-frame on a scene where he exploded out of the gold (right before the Arkenstone was lost). From that picture he had lots of horns - which is great. "Do not come between the Nazgul and his prey"