1)
Why does it mention Aharon?
1.
Redak: He was the head of Kehunah. Since it mentions the Leviyim, it mentions also this Avodah of Aharon u'Vanav - Haktarah, and to atone for Yisrael.
2.
Malbim: "Aharon" refers to the Kohen Gadol, who stands in place of Aharon.
2)
What is "Meleches Kodesh ha'Kodoshim"?
1.
Malbim: It is Avodas Yom Kipur.
3)
How do Kohanim atone for Yisrael?
1.
Malbim: They offer Chata'os and Ashamos.
