Why did Hashem now speak to Aharon as well?
Rashi and Ramban: Because, following Moshe's complaint regarding his speech defect, Hashem sent Aharon together with him to act as his spokesman. 1
What does the Torah mean when it writes that He commanded them regarding the B'nei Yisrael and regarding Par'oh ... ?
Rashi #1: He commanded them to lead Yisrael with gentleness and Par'oh with respect.
Rashi #2 and Ramban #1 (citing the Ibn Ezra): He commanded them with regard to the matters concerning Yisrael and his Shelichus to Par'oh, which the Torah will present later (from Pasuk 29 and onwards).
Ramban #2 (on Pasuk 12): The Pasuk means that Moshe and Aharon should speak to Yisrael and tell them whatever Hashem would command them; and Par'oh to send Yisrael out of his land.
Seforno: Hashem appointed Moshe and Aharon princes 2 over Yisrael and over Par'oh - creating an aura of Divine authority that forced Yisrael and Par'oh would obey their every command concerning the redemption.
Moshav Zekenim (Pasuk 12): It means that Hashem commanded B'nei Yisrael to leave Egypt, and Par'oh to expel them, even if they do not want to go.
Hadar Zekenim: He commanded them for the sake of B'nei Yisrael, that they go to Par'oh and tell him to expel them.
Yerushalmi, Rosh Hashanah, 3:5): Following Pasuk 11, Hashem instructed Moshe and Aharon to teach Yisrael the Dinim concerning sending away one's Eved Ivri after six years. 3
Oznayim la'Torah: Which they must accept without being fazed - a major lesson in leadershhip. And this is followed by the Yichus of Re'uven, Shimon and Levi, the three tribes that produced the three people who caused Moshe the most trouble
What are the connotations of the word "va'Yedaber Moshe el Hashem Leimor" that Moshe added here?