Seeing as Aharon now became Moshe's spokesman, why did Hashem still see fit to assist them personally?
Seforno: So that their word should penetrate Pharaoh's heart and to prevent him from rising up against them and throwing them out of the palace.
Ramban 1 : So that his words should find favor in the ears of all his listeners.
Since Aharon was the chief spokesman, why did He still need to assist Moshe?
As indicated in Pasuk 16 (Ramban).
Ramban: This is due to Kavod Malchus. However, when Moshe repeated his complaint later (See Va'eira, Bereishis 6:30), Hashem relented, and allowed Aharon to act as his spokesman even when addressing Pharaoh. In fact, this was a Kavod for Moshe - as the Pasuk states there (in 7:1) "See, I have appointed you as a G-d over Pharaoh and Aharon your brother will be your prophet".
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 not a Bechor 1 ).
Even if Aharon was a Bechor, perhaps Hashem wanted to make Moshe Kohen anyway. For various reasons, sometimes the Kehunah was not based on birth, e.g. Shem, Yakov, Levi, Moshe (during the Milu'im) and Pinchas! (PF)
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 Chachmah - or, he was Bechor for Nevu'ah. Rashash - Bamidbar Rabah means that he was truly the Bechor, unlike the Midrashim [and almost all Meforshim], which say that Miryam was Aharon's older sister. Rashbam (Shemos 15:20) - Miryam is called Achos Aharon, due to Bechorah. (In Bereishis 28:9, Bereishis 36:22 he wrote 'for Aharon was her oldest brother.' Bamidbar 26:59 - Yocheved bore, to Amram, Aharon, Moshe and Miryam their sister. Several verses list children not in order of birth. Surely Miryam is Moshe's older sister, the youth who called a wetnurse for Bisyah. Can we say that that she was a three month old twin, or that the Torah never mentions that sister again, and Miryam was born after Moshe?! (PF)