All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Opening the coffin and leaving the chest of the dead body uncovered for some time, a day, less or more than that, to give people an opportunity to have a final look at the body is contradictory to the Sunnah and to the consensus of the scholars regardless of whether the dead person is a Mahram to the people who have this look at him or not their Mahram. Rather, it is forbidden for men to look at a non-Mahram woman.
On the contrary, there is clear evidence from the Sunnah that the corpse of the dead person should be covered. ‘Aa’ishah narrated that when the Prophet died, he was covered with a Hibra cloth (a kind of garment which was made in Yemen with a slight green color). [Al-Bukhari, Muslim and Ahmad]
An-Nawawi while explaining this Hadeeth, said: “There is a consensus among the scholars that the corpse of the dead person should be covered. The wisdom behind this is to preserve the body of the dead person from being exposed and to cover him/her from the eyes of the living people.”
But if the relatives of the dead person uncover his face before he is put in the coffin, then this is permissible if this does not cause any delay in preparing the corpse for burial.
As for uncovering the chest, then this is not known in Islam, as we already clarified.
Allah knows best.