So, basically what you gents seem to be saying is that these portrait subjects used our common method - throwing the plaid out on the floor or ground and hand-pleating it - but were more careful about it? Not sure I'm buying that. I don't see how the precise, even pleating shown can be achieved that way.
What I think we perhaps see evidence of is "cheating" by the well-off, who probably did not spend 24 hours a day in their plaids like common clansmen did (unless on campaign). As mentioned previously, there are hints of belt-loops and draw-strings in the primary evidence, as well as servants' helping hands, as UC Guy brought up. I think these "upper crust" Highlanders were availing themselves of these or similar advantages in their desire to look fine!

As for these paintings themselves, I don't see much evidence of artistic license, except of course in the backgrounds. The details of weapons, uniforms and clothing, etc., are almost photographic....