1)
Verse 12 said that they abandoned "Hashem Elokei Avosam." Why does our verse say "they abandoned Hashem"?
1.
Malbim: The name Hashem alone alludes to Him as the Creator. Elokei Yisrael or Elokei Avosam alludes to His Hashgachah over Yisrael. Initially, they served b'Shituf (Hashem and idolatry). This denies Hashem Elokei Avosam (Hashgachah). Afterwards, they descended terribly and denied Hashem the Creator (served only idolatry)!
2)
What are Ashtaros?
1.
Radak: They are images in the form of Ashtaros (female goats or sheep).
