1)
Who sinned?
1.
Radak: He speaks on behalf of Bnei Galus.
2)
What is the meaning of "Im Avoseinu"?
1.
Radak: Just like our fathers sinned, so we sinned.
2.
Malbim: We persisted in our fathers' deeds, and sinned like them. However, we added to their sin. They sinned amidst desire, or b'Shogeg (Chet). We sinned amidst crooked intellect or denial (Avinu) and b'Mezid (Hirshanu).