Having assured Eliezer that an angel would accompany him, why does Avraham then contend with the possibility that the woman would refuse to leave Charan?
Hashem might well assure Eliezer that he would find Charan and Avraham's family successfully, but Rivkah's response lay in her hands and Hakadosh-Baruch-Hu on principle, does not interfere with a person's Bechirah. (EC)
Why did Avraham need to say "If the woman does not want to go... you will be absolved from this oath"?
Rashi: You will be absolved from the Shevu'ah not to take one of the daughters of Aner, Eshkol and Mamrei, 1 (but not from the Shevu'ah of not taking Yitzchak to Charan).
Ramban #1: He would be absolved only from finding a wife for Yitzchak - and Hashem would do as He pleased. He was not permitted to break the other conditions. 2
Ramban #2: He would be absolved from finding a wife from Charan, and would turn to the daughters of Yishmael or Lot. 3
Hadar Zekenim: You will be absolved from going to my homeland, but not from not taking a Kena'anis for my son.
Moshav Zekenim (Pasuk 2): Eliezer did not want to accept an oath with a curse, for a Chacham's curse is fulfilled in every case, i.e. unless he explicitly says when he will be exempt.
Rashi says that if she will not come, Eliezer may take a daughter of Aner, Eshkol and Mamrei. Why could he then not take his own daughter, seeing as they were all Kena'anim?
Da'as Zekeinim #1: We are forced to say that in fact, Aner, Eshkol and Mamrei were not Kena'anim.
Da'as Zekenim #2, Hadar Zekenim #1, Riva (all on Pasuk 39) #1: Whereas No'ach's curse on the Kena'anim (to be slaves) was only potential, 1 Eliezer was factually an Eved, in which case the prohibition remained in place. 2
Da'as Zekenim #3, Hadar Zekenim, Riva #2 (all on Pasuk 39): It would be disgraceful to marry Yitzchak to the daughter of his slave.
Da'as Zekenim #4, Hadar Zekenim #3 (on Pasuk 39, from Bereishis Rabsi 24:34): Eliezer was Cham's son.
Why was Yitzchak forbidden to leave Eretz Yisrael (even in the event that the woman refused to leave Charan)?
Rabeinu Bachye: Avraham did not want him to leave the Holy land 1 because he was an Olah Temimah.
Oznayim la'Torah #1: It was important for the woman to leave her country of birth, not Yitzchak. Refer to 24:3:6:3.
Oznayim la'Torah #2: If Yitzchak were to leave the land that Hashem promised his children, it would be akin to foregoing the inheritance.
Note, a Korban must remain within the precincts of Yerushalayim and sometimes even in the Azarah of the Beis-Hamikdash.
Why does it say "Only do not return my son there?"
Rashi: It comes to extrapolate
Moshav Zekenim: We find that "v'Hayisa Ach Same'ach" includes the last night (Shemini Atzeres) for Simchah! R. Chaim Palti'el (Devarim 16:15) answers that v'Hayisa Same'ach includes the last night, and Ach excludes the first night. The Vilna Gaon answers that it excludes the Mitzvos of Sukah and Lulav (Simchah is the only Mitzvah).
Ha'Emek Davar: Even though Kidushin does not apply to Bnei Noach, someone designated for Yitzchak would conduct with the laws of Yisrael.
What is 'Shevu'asi Zos', from which he will be exempt? Was there another Shevu'ah?
Rashi (according to Bartenura, Gur Aryeh): He will be exempt from the last matter mentioned (in verse 3), i.e. not to take not a daughter of the Kena'anim among whom he lives (Aner, Eshkol and Mamrei). The Shevu'ah not to take from other Kena'anim will still apply.
Rashi (according to Mizrachi), Ramban: He will be totally exempt. It was all one Shevu'ah. 1
Hadar Zekenim: You will be absolved from going to (Ohr ha'Chayim, Malbim
Ramban: Perhaps this included an obligation to go to Avraham's family.