On what basis was Aharon Moshe's interpreter?
Tosefta, Megilah): Because Aharon was Moshe's junior, 1 and whereas it is proper for a junior to act as the interpreter of his senior, the other way round is not.
See Torah Temimah, 6:27, note 19.
What is the meaning of "I made you Elokim to Par'oh"?
Sh'mos Rabah (Sha'anan), Midrash Tanchuma, Mechilta d'R. Yishmael Masechta d'Pischa 1 and Yalkut Shimoni: It means 'You are a judge to Par'oh. 1
Hadar Zekenim, Da'as Zekenim: You are like G-d to him, because Par'oh made himself a god, and said "the river is mine, and I made it." 2
Hadar Zekenim (12:12): 'Elohim' here means a great person, like "u've'Chol Elohei Mitzrayim E'eseh Shefatim." 3
Targum Onkelos: 'I have made you a master over Par'oh'. 4
Targum Yonasan: I have put your fear into Par'oh as if you were his god. 5