Here is Linda's photo superimposed on my previous collage. I interpret my mystery duckling as a significantly younger Hooded Merganser duckling than Linda's. As I found previously on the internet, as they age both their bodies and their bills elongate from a 'duck' format to a 'merganser' format. And the bicoloration of the head fades.
The bird I photographed was not as far along as Linda's (i.e. was younger).
Linda had this bird stay around on her pond a long time after previously being associated with a Hooded Merganser mother so she's sure of the identity.