Seeing as the people approached Moshe with their complaint, why did Hashem give Aharon the answer?
Rashi: Hashem actually told Moshe to pass on the message to Aharon. 1
What is the difference between the two commands that Hashem issued here to Aharon, one to "Atah u'Vanecha u'Veis Avicha", the other to "Atah u'Vanecha"?
Rashi and Seforno: The first command concerns the B'nei K'has, the father of Amram, with regard to a Tamei or a Zar going beyond his permitted border 1 guarding the holy vessels - the Ohel, the Aron, the Shulchan and the holy vessels - against Zarim touching them. The second concerns the Kohanim, with regard to preventing the Levi'im (Zarim - Seforno) from encroaching on the Avodas Kehunah.
Which "sin of the Mikdash" is Hashem referring to?
Rashi: It is referring to the sin of Zarim 1 performing the Avodah that has been allotted to them (to the Levi'im).
QUESTIONS ON RASHI
Rashi writes that Hashem spoke to Moshe, and he should tell Aharon. If so, the entire Torah should say "El Moshe El B'nei Yisrael", for He told Moshe to tell B'nei Yisrael"!
Hadar Zekenim (Vayikra 1:1, citing Sefer ha'Gan): If not for the exclusions, I would say that Hashem spoke to both of them. The exclusions teach that only Moshe heard, but also Aharon was a Shali'ach for the matter.
Hadar Zekenim: This is difficult, for we must delete from the text in Sifra 'to tell Aharon.'
Rashi writes that Hashem spoke to Moshe, and he should tell Aharon. What forced him to say so?
Moshav Zekenim: In 13 places, it says that Hashem spoke to Moshe and Aharon; there, Aharon was mentioned for his honor. Here it says only that He spoke to Aharon.
Chizkuni: Here, va'Yomer is not like elsewhere, rather, He already said about Aharon. I.e. you need not worry that anyone who approaches will die, for Hashem already told Moshe about Aharon "you and yous sons and your father's house (the Leviyim, will bear the sin of the Mikdash, i.e. Zarim doing Avodas ha'Leviyim).