If you do your homework correctly you’ll see that Reb Moshe’s heter allowing a tefach of hair on an eishes ish to be visible was never meant for the general public. It was a psak for a single instance for a woman, who was not living in a frum community, who otherwise would have refused to cover her hair altogether.
There is a letter from R’ Moshe’s son, ( I believe R’ Reuvan) which is published in R’ Falks Sefer (Oz Vehadar Levusha) explaining the premise for that psak. And over there her reiterates that it was not intended as a heter for anyone else.