Is it permissible to address the deceased or absent living with phrases like "Peace be upon you," "May Allah have mercy on you," or "May Allah cure you"? And what is the correct formula for greeting the Prophet (peace and blessings be upon him) and his two companions when visiting his noble grave?

There is no harm in addressing the deceased with the second-person pronoun, such as saying: "May Allah have mercy on you," or "May Allah forgive you." Such expressions have been reported in the hadiths and were used by the Companions and the Successors. An example of this is what the Messenger of Allah, peace and blessings be upon him, did when he entered a grave at night and said: "May Allah have mercy on you; you were indeed oft-returning [to Allah], a reciter of the Qur'an." There is also no harm in addressing an absent living person with the second-person pronoun as if they were present. The Arabs used this in their poetry.