Mod 42, just the opposite; the source for hair covering mid’Oraisa is the fact that until then, her hair was covered. Sotah is a unique case.
The fact that the standards are different for married and unmarried women has been used for the halachos of tefach (R’ Moshe) and krias Shema (Aruch Hashulchan), but never by mainstream poskim for actual heter.
Be careful with limud z’chus; it’s nice to be melamed z’chus on those who didn’t or don’t, but you don’t want to be machshil anyone into not covering their hair based on the notion that there’s solid halachic basis, when there really isn’t.