Why did Avimelech need to make a B?ris with Yitzchak, and ask him to swear, seeing as Avraham had already made a B?ris with him, a B?ris that covered three generations?
Ramban (in 26:29) #1 and Moshav Zekenim: Because he was worried (not that Yitzchak would attack him, but) that, because he (Avimelech had abrogated the covenant that they had made with Avraham (by expelling him from G?rar 1 ), Yitzchak's descendents would do likewise and drive his children out of their land. So he entered into a new covenant with Yitzchak. 2
Ramban (in 26:29) #2: Avimelech entered into a covenant with Avraham, because he was afraid of Avraham, who had proven himself to be a mighty warrior on whose side Hashem fought, and who had a household consisting of three hundred swordsmen who might attack him. Moreover, he may well have heard about Hashem's promise to give Eretz Cana'an to Avraham's offspring. And it is for the same reason that he entered into a new covenant with Yitzchak - who, like his father, was as great as his father, and of whom he likewise feared.
Ohr ha'Chayim (in 26:26): Avimelech was afraid that Avraham had released his oath, which is permmitted even Lechatchilah not in front of the one to whom he swore, provided he did not benefit from him. 3 He offered Avraham to stay there, but he declined. Yitzchak, on the other hand hand, remained and benefited, so he would only be permitted to release his oath in front of Avimelech.
Ha'amek Davar (in 26:26): Expelling Yitzchak from G?rar seemed like abrogation of the B?ris with Avraham. Avimelech came to explain that he did not know that they maltreated him. Had he known, he would have protested.
Ohr ha'Chayim: We do not find that Avimelech stipulated with Avraham that he and his sons may stay in his land. Expelling Yitzchak did not abrogate the B?ris! See 20:15.
Playing down the fact that he had abrogated the original one with smooth talk ('We did only good with you!')
Ohr ha?Chayim: And the gifts given to Sarah were not benefit; they were due to shaming Sarah.
Why does the Torah see fit to mention that Avimelech and his entourage left in peace?
Sifrei: To teach us that rebuking leads to peace.
Ha'amek Davar: Yitzchak did more than they requested, not to harm them. He accompanied them, and added love and Shalom.
What did the oath that they made to each other incorporate?
Targum Yonasan: It incorporated a covenant - which entailed them cutting the bridle of a donkey in half and giving one half to the other as testimony. 1
Targum Yonasan: Following this, Yitzchak Davened on behalf of Avimelech and the P?lishtim.
Why did they rise early?
Ha'amek Davar: It was lest others know about this.
Regarding Avraham, it says "va'Yishbe'u Sheneihem." Why does it say here "Ish El Achiv"?
Ha'amek Davar: Here, all the nobles swore.
