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
This is one of the thirteen places where this occurred, as Rashi explained at the beginning of Vayikra - Sifsei Chachamim. Refer to Vayikra 1:1:154:1 & 2 and note.
What is the difference between the two commands that Hashem issued here to Aharon, one to "Atah u'Vanecha u'Veis Avicha", the other only 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 - Seforno) 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.
Targum Yonasan: Both with reference to the Kohanim regarding the Korbanos - the first with regard to the Avodas ha'Dam, the second, with regard to the Chazeh ve'Shok which they received from the Yisrael's Korban. 1
Based on the Sifri. See Na'ar Yonasan.
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).
Whose sin they carry should they come close, as is the way of Tzadikim. See Rashi in Yeshayah, 53:11
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", since He told Moshe to tell B'nei Yisrael"?
Hadar Zekenim (in Vayikra 1:1): Hashem spoke to Moshe, but Aharon was able to hear. 1
Hadar Zekenim (in Vayikra 1:1, citing Seifer ha'Gan): If not for the exclusions, we would say that Hashem spoke to both of them. The exclusions teach us that only Moshe heard, but also Aharon was a Shali'ach for the matter.
Hadar Zekenim: This is difficult, since we must delete from the text in the Sifra 'to tell Aharon.'
Rashi writes that Hashem spoke to Moshe, and he should tell Aharon. What prompted him to say this?
Moshav Zekenim: In thirteen 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: "Vayomer" here is not like elsewhere; rather, He already said about Aharon - 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 Levi'im, will bear the sin of the Mikdash - Zarim doing Avodas ha'Levi'im).
