What words do they speak?
Radak: Yaravam [ben Nevat] and his colleagues took counsel how to solidify the kingship in his hand. They spoke words that the nation should not ascend to the Beis ha'Mikdash in Yerushalayim, rather, to make images to serve them.
Radak (5, citing Targum Yonasan): Words of sin.
Malbim: They spoke among themselves words of rebellion.
What are "Alos Shav"?
Rashi #1: It is swearing falsely. Alos is present tense, like "Karos". Radak (5) - it is Makor (like a gerund), like Re'os, Rabos.
Rashi #2: Every Bris that they make with one another, they transgress it.
Radak: They swore that the nation will not ascend to the Beis ha'Mikdash. This is Shevu'as Shav, to abolish the Torah's words and Hashem's Mitzvah.
Malbim: Via their words of rebellion, they transgressed the Alah that they swore to their king to be loyal to him. They said that the curse is not binding, since he nullifies Avodah of the calves.
With whom do they make a Bris?
Rashi #1: It is with the idolatry of Molech.
Rashi #2: With one another.
Malbim: The rebels made a Bris or conspiracy to kill Shalum, or to hand over Hoshe'a to Melech Ashur, to kill him.
Why does it say "u'Farach ka'Rosh Mishpat"?
Rashi #1: Afflictions and punishments will fly upon them like Rosh, which is a bitter herb.
Rashi #2: Just like Rosh is a bitter herb, their judgments increase bitterness for the poor. So Amos said "ha'Hofechim l'La'anah Mishpat" (5:7).
Radak: Because they spoke these words (refer to 10:4:1:1), Mishpat flew on them like Rosh v'La'anah. This is like "Shoresh Poreh Rosh v'La'anah" (Devarim 29:17).
Malbim: The 'seed' of rebellion that they planted in the hidden furrows of their hearts, it sprouted and made bitter fruit clusters.
Why does it say "Al Talmei Sadai"?
Rashi #1: The punishments will fly upon them like Rosh flies over furrows in a field.
Rashi citing Targum Yonasan: It is due to the sin of moving the borders of fields (to steal land). Telem is a plow furrow.
Rashi #2: They make their Mizbechos in the furrows - "Gam Mizbechosam k'Galim Al Talmei Sadai" (12:12).
Radak: This is a Mashal for the evils that will come upon them to punish them for this. The coming verse proves that this is correct. Sadai is like Sadeh.
Sha'arei Teshuvah 1:27: Resha'im made pride a plowed furrow in their hearts, and seed in it evil thoughts, to sprout Aveiros - the fruits of their thoughts.
Malbim: Their punishment was immediate and exposed.
