Why did Hashem include Aharon in this Dibur?
Rashi: Since Aharon exerted himself together with Moshe in performing the miracles, Hashem gave him the Kavod of including him in the first Mitzvah that he was teaching Moshe. 1
Refer to 12:2:3:1.
Also see 11:10:1:1 and 11:10:1:3. Mechilta - We learn from the Pasuk in Parshas Va'era (6:28), that throughout the entire Torah, Hashem always spoke to Moshe exclusively, and it is only in three locations that He spoke to Aharon as well. See Torah Temimah (citing the Vilna Gaon) who presents the three locations.
When was this Dibur said?
Rashi: Just before Rosh Chodesh Nisan. 1
Refer to 12:2:1:1 and its note. But see 12:2:1.2:3.
Why does the Torah need to mention that it was said, "in the land of Egypt"?
Ramban #1 and Rashbam: Refer to 12:2:4:1*.
Ramban #2: To teach us that Hashem spoke to Moshe and Aharon 'in the land of Egypt' -- not 1 in the town. 2
Moshav Zekenim: This is unlike the Midrash that says that this is one of 10 places in which Tahor was revealed in a Tamei place.
Why did Hashem add the word "Leimor"?