Does the Indian-like plain white feather belong to a Scottish hat, or is it as incongruous as the jabot and the male beret she wears?
A single eagle’s feather is normally worn by the son of some clan chieftains (who might themselves have three), I think, but it is not in and of itself unusual to see a feather - often from a pheasant or grouse - or other decoration (Grants wear a sprig of Scot’s Pine, for example), behind a clan crest, regimental badge or whatever. You can see that in these pictures
, and here too
, and for comic effect here
As for a girl wearing a feathered bonnet, see this illustration
. I think the effect that Hergé was aiming for was along the same lines as shown here
; and no, Scotland isn’t famous for its oranges
, but I don’t suspect a spatio-temporal fault… ;-)
About Highland Dancing: at what period did Scottish girls and women start to practice these men's dances?
Don’t know, I’m afraid; they may always have done it in some places, but the dance is based on male display.
Update: This site
doesn’t say when women first danced the Highland dances, but it does say that the first girl to dance in competition
was Jenny Douglas, late in the 19th Century
; so it has been over one hundred years at least…