How is the speaker in the verse: "And I am not a guardian over you" (وما أنا عليكم بحفيظ) the Messenger of Allah, peace and blessings be upon him, even though the Quran is the word of Allah, and it does not contain "Say" (قل) or anything similar, and what is the exegetes' evidence for that?

The command "Say" is understood at the beginning of the verse, which is common in the Qur'an. This represents a transition from refuting the polytheists to affirming God's Oneness. The discourse resumes with an address directed to the Prophet, peace and blessings be upon him, presented as the subject of an omitted verb of command to speak. An example of this is Surah Al-Fatihah, where "Say" (plural) is understood before "Alhamdulillah" (All praise is due to Allah), as evidenced by the verse: "It is You we worship." The style of the discourse indicates this, as Arabs omit words when their place is known and indicated by the apparent context of the speech, especially if the omitted word is a statement or the interpretation of a statement.