Why did Avimelech need to come to make a Bris with Yitzchak, and ask him to swear? Avraham already swore, and his Bris covered three generations!
Ramban (to 26:29) #1, Moshav Zekenim: 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 Gerar 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 (to 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 HaSh-m fought, and who had a household consisting of 300 swordsmen who might attack him. Moreover, he may well have heard about HaSh-m's promise to give Eretz Kena'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 (to 26:26): Avimelech feared that Avraham permitted his Shevu'ah. One may do so even l'Chatchilah not in front of the one to whom he swore, if he did not benefit from him. 3 The gifts given to Sarah were not benefit; they were due to shaming Sarah. He offered Avraham to stay there, but he did not stay. Yitzchak stayed there and benefited, so he could permit his Shevu'ah only in front of Avimelech.
Ha'amek Davar (to 26:26): Expelling Yitzchak from Gerar seemed like abrogation of the Bris with Avraham. Avimelech came to explain that he did not know that they mistreated 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 Bris!
Playing down the fact that he had abrogated the original one with smooth talk ('We did only good with you!')
Ohr ha'Chayim learns from Sotah 36b (Pharaoh had made Yosef swear not to reveal that Pharaoh does not know Hebrew. Later, he told Yosef to permit his oath to Yaakov. Yosef said, if so, I could permit also my oath to you! Also cited in Rashi to 50:6.) I do not understand his proof. Yosef said, if you hold that I can permit the latter, then I can permit also the former! Yosef received awesome benefit from Pharaoh. Some say that it depends on benefit due to the Shevu'ah (Rema YD 228:20). However, it seems that this is never permitted l'Chatchilah (YD 228:20). (PF)
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 Shevu'ah 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
Following this, Yitzchak prayed on behalf of Avimelech and the Pelishtim and HaSh-m healed them; and they left in peace.
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.