For whom were the Bamos?
Malbim: Most of them were for Shamayim. It was forbidden to offer on them, for they are outside the Mikdash. Chizkiyah destroyed them and Menasheh built them, to cause the Rabim to sin.
Why does it say here "ha'Bamos Asher Ibad", and in Divrei ha'Yamim II, 33:3 "ha'Bamos Asher Nitatz"?
Malbim: Here it discusses Bamos for Shamayim. Divrei ha'Yamim discusses Bamos for idolatry; they require Netitzah (demolition).
Why does it say here "Ba'al", and "Asherah" in the singular, and in Divrei ha'Yamim II, 33:3 it says "Be'alim", and "Asheros" (plural)?
Malbim: Here it says "like Achav did", and Achav made only one of each 1 . Ba'al is for the sun, and Asherah for the moon. In Divrei ha'Yamim it teaches that he added to Achav, and made many. There it does not say 'like Achav did.'
Sanhedrin 113a says that Achav put idolatry in every furrow! Did he make only one Ba'al, and countless of other idolatries? Melachim I, 16:32-33 implies that Ba'al was his primary idolatry! (PF)
Why is he compared to Achav?
Radak: Achav began to make Asherah, among Malchei Yisrael. Before him they did not make Asherah; they merely went in [the way of] Chatas Yaravam.
Sanhedrin 103a: He was like Achav, who was an evil king for 22 years. Since Menasheh reigned 55 years, this teaches that he repented for [his last] 33 years.
Why does it mention bowing and serving?
Malbim: Bowing could be merely for the sake of honor, if he views them as intermediaries between [him and] Hashem, just like we honor the king's officers. However, serving proves that he considers them to be deities.
