Untying the knots and exposing the face of the deceased when burying him

It is mustahabb to untie the knots in the shroud of the
deceased after burying him, because they are only tied lest the shroud fall
open. Once he has been placed in the grave, they may be undone. 

Ibn Qudaamah said in al-Mughni (2/191): 

With regard to undoing the knots at his head and feet, it is
mustahabb, because they were tied lest the shroud fall open, but that is no
longer a concern once he has been buried. It was narrated that when the
Prophet (blessings and peace of Allaah be upon him) placed Nu’aym ibn
Mas’ood al-Ashja’i in the grave, he removed the ropes using his teeth. And a
similar report was narrated from Ibn Mas’ood and Samurah ibn Jundub. End
quote. 

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) said in
Majmoo’ Fataawa Ibn ‘Uthaymeen (17/183): With regard to undoing the
knots in the wrappers, there is a report concerning that from ‘Abd-Allaah
ibn Mas’ood (may Allaah be pleased with him) who said: When you have placed
the deceased in the grave, undo the knots. 

With regard to uncovering the face of the deceased
altogether, there is no basis for that. The most that has been narrated
concerning it — if it is saheeh – is that ‘Umar ibn al-Khattaab (may Allaah
be pleased with him) said: When I die and you place me in my grave, let my
cheek touch the ground. End quote. 

And Allaah knows best.