1)
Why does it mention Avon and Chet?
1.
Malbim: Avon is due to denial. This was a wall that totally separated [you from Him]. It blocks Hashem from coming close to you, from His side and from your side - "Beinechem l'Vein Elokeichem." Chet is due to desire, not due to denial. It does not separate between man and Hashem, just it hides His face, that He does not hear your cries.
2)
Why does it say "Histiru Panim Mikem"?
1.
Rashi: They caused that He hid His face from you. Radak - this is Po'el Yotzei Shelishi (it causes a second party to act on a third).
