Why is the phrase "Say, O Muhammad" repeated in English translations of the meanings of the Qur’an, and are the verses that begin with "Say" addressed solely to the Prophet, peace and blessings be upon him, or to us, or to both?

The translation you provided for the meaning of the verse is correct, because the address, using the word (Say), is directed solely to the Prophet, peace and blessings be upon him. His Lord instructs him in His argument and commands him to convey His revelation. This emphasizes that the Prophet had no role in formulating the Qur'anic text; rather, he followed the revelation, conveying it exactly as he was commanded. The phrase (Say) has been repeated more than three hundred times in the Qur'an to clarify that Muhammad, peace and blessings be upon him, had no input into the revelation; he was addressed, not speaking on his own behalf, narrating what he heard, not expressing anything from within himself.

The address directed to the Prophet, peace and blessings be upon him, whether using the word (Say) or otherwise, falls into three categories: a. Specific to him, with no one else sharing in it. b. General to him and others, due to its guidance on disputing with adversaries or shared obligations. c. Specific in its wording but general in its ruling, where there is no evidence for its specificity or generality.