1)
Why does it say "Shavu Al Avonos Avosam ha'Rishonim"?
1.
Radak: This refers to the generations of Amon, Menasheh and Achaz.
2.
Malbim: Previous generations sinned only passively. This generation is worse; they actively went after other gods to serve them. Not only did they not celebrate Hashem's Chagim or give Terumos and Ma'aseros - they made Chagim for idolatry, and gave Terumos and Ma'aseros to priests of the gods.
2)
Why does it say "Heferu
1.
Malbim: Not only did they not fulfill My Bris - they totally annulled it, since they chose other gods.
