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 Pharaoh ... ?
Rashi #1: He commanded them to lead Yisrael with gentleness 1 and Pharaoh 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 Pharaoh, which the Torah will detail 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 Pharaoh to send Yisrael out of his land.
Seforno: Hashem appointed Moshe and Aharon princes 2 over Yisrael and over Pharaoh - creating an aura of Divine authority that forced Yisrael and Pharaoh would obey their every command concerning the redemption.
Moshav Zekenim (verse 12): He commanded Bnei Yisrael to leave Egypt, and Pharaoh to expel them, even if they do not want to go.
Hadar Zekenim: He commanded them for the sake of Bnei Yisrael, that they go to Pharaoh and tell him to expel Bnei Yisrael.
Why do we find "va'Ydaber Moshe" to Hashem only here, and in Bamidbar (27:15)?
Hadar Zekenim: Here, Moshe asked whether or not Hashem will redeem [Yisrael]. There, he asked whether or not He will appoint a leader over Yisrael [for after Moshe dies].