I actually have less problem with the cross-dressing girl in the book I've just read, Cue for Treason, because as well as all the points you mention it's established that she's an excellent actor. I don't recall that Gwyna has any such talent or training, despite her keen observation of people. In fact, as a timid girl - Gwyna the mouse, she calls herself at the start - she might seem an unlikely person to be able to pass as a boy in the macho world of the Arthurian warband. Why do you think it would be easier for her to pass then than it would be for her modern equivalent now?

Really, though, and despite the paragraph above, I don't have a 'problem' with it at all: but I do see its plausibility being at least lubricated (is that a good word? Not so sure...) by my awareness of the generic convention.

All this is a bit of a side-issue, admittedly!