Why does it say "Shim'i ha'Aretz"?
Radak: The land is a witness between me and you. He did not call Shamayim to be a witness, because the land will be desolate due to their sins 1 .
If so, the testimony affects the land! Indeed, for a judgment in Beis Din, one cannot testify even if this is detrimental for himself. Here, it shows even better that it is true - why would the land lie to harm itself?! (PF)
What is "Pri Machshevosam"?
Rashi: It is the reward of their thoughts 1 .
Radak: He is not punishing for mere thought, rather, for deeds that are the fruit (result) of their thoughts. They think that I do not see their deeds, therefore they follow their hearts to fulfill their desires!
Radak citing his father: It is intent [to sin] that a person was unable to fulfill. However, if he refrained due to Yir'as Shamayim, they do not join to deed [to be punished for them].
Radak citing Targum Yonasan: Payment for their deeds.
Malbim: The punishment is due to casting off the yoke - they had thoughts of heresy.
Kidushin 40a: Normally, Hashem punishes for a bad intention only if it is fulfilled. Then, He punishes (additionally) for the intent. "In order to trap Bnei Yisrael in their hearts" refers to idolatry; it is an exception, for one who denies idolatry is like one who admits to the entire Torah.
Why does it say "Al Devari"?
Radak: This is like El Devari, just like "va'Yelech Elkanah Al Beiso" (Shmuel I, 2:11)
Why does it say "v'Sorasi va'Yim'asu Vah"?
Rashi: This is like "Avi v'Zimras Kah va'Yhi Li li'Yshu'ah" (Shemos 15:2; it says va'Yhi in place of Hayah). Also here, it says va'Yim'asu in place of Ma'asu. Verses speak like this.
Radak: This is like v'El Torasi Lo Hikshivu. Devari refers to the words of the Nevi'im, who warned them. Torasi is Toras Moshe. Not only did they not listen when they read it to them - they also despised it, and do not want to read it and hear it. In the days of Yoshiyah, when Chilkiyah found the Sefer Torah and read it, it was a Chidush to them, for they already forgot it.