What is the evidence of those who say that the 'awrah (intimate parts) of a Muslim woman in front of non-Muslim women is the entire body except for the face and hands?

What you referred to, regarding a Muslim woman's awrah (nakedness) in front of a non-Muslim woman being her entire body except for the face and hands, is the view of the majority of jurists. Some of their evidence is the Almighty's saying: "Or their women," meaning Muslim women, and a non-Muslim woman is not among them. Also, because a non-Muslim woman might describe the Muslim woman to her non-Muslim husband. The Hanbalis held the view that there is no difference between a Muslim woman and a Dhimmi (non-Muslim living under Muslim rule) woman concerning looking. The Shafi'is permit a non-Muslim woman to see what is apparent of a Muslim woman during work. Others hold that it is permissible to see what a Muslim woman sees of her, due to the unity of gender.