Some interesting questions coming up, not all of which I know the answer to. But regarding grey flanks, I think this is a feature of adult males in some eastern taxa (notably baicalensis and lugens).
Baicalensis (in winter plumage) is a grey mantled taxon, with an extensive breast patch, typically meeting the bend of the wing and extending up in an arc onto the rear ear coverts. There is often a narrow line from the bill base extending onto the ear coverts, and a dark shadow across much of the rear ear coverts. So, pretty much like the Po Toi birds, really.
As per the adult male in the photo John refers to, an extensive white forecrown is typical of baicalensis.
