Intriguing bird for sure. I don't have too much of a problem with the darkness of the coloring or patterning of the head, as Canvasbacks are quite dark; and the slight mottling/flecking of the feathers near the base of the bill and perhaps elsewhere on the head also don't cause me any concern. Many of them could be accounted for by moult.
I do, however, feel the bill shape is beyond any normal Canvasback. The shape is definitely more concave and, especially near the tip, more narrow than any Canvasback I've ever seen. (I live in the Canadian Prairie Province region and have seen many individual Canvasbacks.) The bill on all Canvasbacks I've seen make pretty much a straight line with the forehead, if not actually slightly convex. This is a feature that really doesn't vary much. And, in the third photo (first large one), I see what appears to be the characteristic Pochard dark-tipped bill (contrasting with a slightly paler rest of bill). Combined with the other anomalies you have pointed out, I think you are wise in showing reluctance in calling this a Canvasback. I confess, though, if I were out birding in my local patch I could easily pass over this bird!