“Covering hair was a neglected mitzvah as was shatnez.”
I fully cover my hair, so this reply does not come from a place where I feel I shouldn’t have to do so. However, you are comparing the issur of shaatnez which is clearly written in the Torah Shebichsav, to the issur, as we understand it, of a married woman uncovering her head, which is only implied in the very barest (no pun intended) of ways in the Written Torah, and not spelled out b’feirush. Even in the Gemarah where it discusses Seyar B’isha Erva, it is the subject of much discussion as to what constitutes seyar, erva, how much, what may be seen, etc. The implication because the Sotah’s head is uncovered by the Kohein, is that it must have been covered up. You probably cannot really compare these two ideas, to make your point, IMO. They would appear to be apples and oranges.