Seeing as Aharon now would become Moshe's spokesman, why did Hashem still see fit to assist them personally?
Seforno: So that their words should penetrate Pharaoh's heart, and to prevent him from rising up against them and throwing them out of the palace.
Ramban: So that his words should find favor in the ears of all his listeners. 1
Since Aharon was the chief spokesman, why did He still need to assist Moshe?
Ramban: As indicated in 4:16.
Ramban: This is due to Kavod Malchus. However, when Moshe repeated his complaint later (6:30), Hashem relented, and allowed Aharon to act as his spokesman even when addressing Pharaoh (7:2). In fact, this was a Kavod for Moshe - as the Pasuk states "See, I have appointed you as a G-d over Pharaoh, and Aharon your brother will be your prophet" (7:1).
QUESTIONS ON RASHI
Rashi writes that Aharon was supposed to be the Levi, and Moshe, the Kohen. How could Moshe be a Kohen? Kehunah depends on Bechorah, and Aharon was the Bechor!
Tosfos ha'Shalem (6): Yocheved miscarried before Aharon was born (so also Aharon was [also] not a Bechor 1 ).
Even if Aharon was a Bechor, perhaps Hashem wanted to make Moshe a Kohen anyway. For various reasons, sometimes the Kehunah was not based on birth, e.g. Shem, Yaakov, Levi, Moshe (during the Milu'im) and Pinchas! (PF).
The Kehunah was synonymous with Bechorah only until Aharon was chosen - even though he was not a Bechor. From then on, his descendants were Kohanim, irrespective of whether they were Bechorim or not.
Bamidbar Rabah 6:2, Tosfos Chachmei Angliyah and Maharsha (both on Sanhedrin 7a): Aharon was the Bechor. The Ta'amim for Bamidbar 3:2 connote that Aharon was the Bechor. Ba'al ha'Turim - He was Bechor with respect to Moshe (and so Etz Yosef explains Bamidbar Rabah). Meshech Chochmah - Or, Aharon was Bechor for Nevu'ah. Rashash - Bamidbar Rabah means that he was truly the Bechor. This is unlike the Midrashim [and almost all Mefarshim], which say that Miriam was Aharon's older sister! Rashbam (to Shemos 15:20) - Miriam is called Achos Aharon, due to Bechorah. (In Bereishis 28:9, Bereishis 36:22 he wrote 'for Aharon was her oldest brother.' "Yocheved bore to Amram, Aharon, Moshe, and Miriam their sister" (Bamidbar 26:59). Several verses list children not in order of birth. Surely Miriam is Moshe's older sister, the youth who called a wet-nurse for Bisyah (2:7-8). Can we say that that she was a three-month old twin, or that the Torah never mentions that sister again, and Miriam was born after Moshe?! (PF))
