Does not the pronoun "na" in "ihdina" (guide us) in Surat Al-Fatiha indicate the plural, even though the speaker is Allah?

The summary of the answer is that the plural form in Al-Fatihah, "Guide us" (اهدنا), is not due to the servant's ignorance that Allah is the One being invoked. Rather, it expresses the servant's acknowledgment of their poverty and need for Allah Almighty, along with the community of believers. This style aligns with the context of the surah, which begins with praise and monotheism, then shifts to seeking guidance. Here, the supplication using the plural pronoun "Guide us" (اهدنا) is more majestic and humble, as it is an acknowledgment from the servant that they are part of a large group of servants who are in need of Allah, and not merely an isolated individual. Furthermore, Al-Fatihah is the word of Allah, which He commanded His servants to recite in the plural form to emphasize belonging to the community of believers and seeking guidance and assistance from them.