Is it impermissible to pause between the two salutations at the end of the prayer, and what is the guidance of the Prophet, peace and blessings be upon him, regarding that?

The established practice from the Prophet's guidance (peace be upon him) is to give the salutation to his right until the whiteness of his right cheek is visible, and to his left until the whiteness of his left cheek is visible. It is also authentically reported that he would sometimes give a single salutation straight ahead, inclining slightly to the right side. Imam An-Nawawi said: "If we hold the view of two salutations, then one is to his right and the other to his left, and he begins the salutation facing the Qibla."

He begins each of the two salutations facing the Qibla. Pausing between them for longer than it takes to face the Qibla is not permissible, as it constitutes an addition to the prayer.