What is the significance of the "Amim" and the "Le'umim" to whom Yitzchak referred?
Seforno: All the nations would serve him, whereas even the kingdoms, who were not destined to serve him, would bow down to him.
Malbim: Le'umim are nations with a creed. They will not serve you; they will only bow to you.
Ha'amek Davar: Am is a nation under a different kingdom. Le'umim (independent nations) will be submissive to you. 'Yishtachavu' is written Chaser (without a Vav), for they will not bow like slaves, rather, to honor you.
Since, without realizing it, Yitzchak was blessing Yaakov. In his mind however, he probably meant that the nation of Yisrael would bow down to Esav (Refer to 27:29:2:1).
Targum Yonasan also adds that 'whoever cursed him would be cursed like Bil'am ben Be'or, and whoever blessed him would be blessed like Moshe the Navi of Yisrael.'
Why did Yitzchak not insert the Berachah of Avraham and of Eretz Yisrael in the current Berachos?
Seforno: Because he thought that he was blessing Esav, 1 and, although he did not realize the extent of Esav's evil, he knew that the Berachos of Avraham (predominantly that he would inherit Eretz Yisrael), belonged to Yaakov. 2
Why did Yitzchak intend to bless Esav that he should be master over Yaakov?
Seforno: He thought that it would suffice for Yaakov to inherit Eretz Yisrael, while at the same time being slightly subservient to his brother Esav. 1
Why did Yitzchak refer to "Bnei Imecha," whereas when Yaakov blessed Yehudah, he referred to "Bnei Avicha" (49:8)?
Rashi and Rashbam: Because whereas Yitzchak had only one wife, Yaakov had four - and he was blessing Yehudah that all of his (Yaakov's) sons would bow down to him, and not just the sons of his mother.
Why did Yitzchak put the curse first? HaSh-m forced Bil'am to say the Berachah first - "Mevorachecha Varuch, v'Orerecha Arur" (Bamidbar 24:9)!
Rashi: Because Tzadikim (such as Yitzchak) first experience suffering, and enjoy tranquility only at the end; by the same token, those who curse them precede those who bless them. Whereas by Resha'im, the order is reversed. Hence Bil'am (ha'Rasha) reversed the order, placed Berachah before Kelalah.
Seforno: "Orerechah Arur" actually refers to the previous Berachah, "Be a master over your brother," seeing as there are many slaves who curse their masters, whenever their wishes are not met. 1
Chizkuni: (Normally, one should begin with the Berachah.) Yitzchak thought that he was blessing Esav, and he knew that many hate him, so he began with cursing those who curse him. We find that HaSh-m blessed Avraham "u'Mekalelcha A'or" (12:3 - in the singular), for he had few enemies.
R. Chaim Paltiel: Tzadikim reverse Midas ha'Din to Midas ha'Rachamim (so they begin mentioning the curse). Resha'im reverse Midas ha'Rachamim to Midas ha'Din (so they begin mentioning the Berachah). HaSh-m always begins with the good.
Malbim: When the blessings are meant for Yisrael, they begin with a Berachah of Hashgachah Pratis for those who bless Yisrael; one who does not bless them, and all the more so one who curses them, is cursed. Yitzchak could not say so about Esav. Rather, HaSh-m will avenge those who curse him, and automatically, one who blesses will be blessed. 2
What are the ramifications of "Orerecha Arur, u'Mevarechecha Baruch!"?
Yerushalmi (Berachos 8:8): When a Nochri met R. Yishmael and blessed him, the latter responded with 'Your reply has already been stated;' and when another Nochri met him and cursed him, he gave him the same response. When his Talmidim queried him on this, he quoted them the Pasuk, "Orerecha Arur u'Mevarechecha Baruch!"
Ohr ha'Chayim: If one curses with intent for a Berachah (like in Mo'ed Katan 9b), he is blessed.
Why does it say both "Gevir l'Achecha" and "v'Yishtachavu Lecha Bnei Imecha"?
Ohr ha'Chayim #1: Esav himself will not be your slave, but you will be a Gevir (superior) to him. His children will bow to you.
Ohr ha'Chayim #2: If you are Misgaber in Kadosh service, they will bow to you, but not if you slacken.
Ha'amek Davar: You will be superior to Bnei Esav who live in Se'ir, They are the primary Edom; only they are called Achicha. They were submissive even to Achav and evil kings of Yehudah, due to great Chesed (that Bnei Yisrael did). "V'Yishtachavu Lecha Bnei Imecha" refers to the nation that assumed the name Edom during Bayis Sheni. It was true slavery, until Yisrael ceased to do Chesed, and there was groundless hatred and murder.
Why are "Orerecha" and "Mevarechecha" plural, but "Arur" and "Baruch" are singular?
Ohr ha'Chayim: Not everyone who curses a Yisrael is cursed (e.g. Achiyah ha'Shiloni), and not everyone who blesses a Yisrael is blessed (e.g. Bil'am).
QUESTIONS ON RASHI
Rashi writes: "Although Yaakov [later] said 'your father's sons' (49:8)... in this case, since Yitzchak had only one wife, he said, 'your mother's sons.'" But Yitzchak too could have said 'your father's sons,' as we usually call children by their father's lineage?
Gur Aryeh #1: Yitzchak blessed Yaakov that his brothers should be subservient to him, just as his mother is subservient to her husband.
Gur Aryeh #2: Yitzchak did not wish to associate this curse of subservience with himself. 1
Chizkuni also proposes an answer, but Gur Aryeh disputes it.
Rashi writes: "The wicked have tranquility at the outset... that is why Bil'am mentioned blessing first." But Bil'am was talking about the Bnei Yisrael; he should have used the same order as our verse!
Gur Aryeh: Blessing comes to the recipient through the giver; Bil'am blessed Yisrael in the order that reflected his own nature.
Rashi writes that Yitzchak put the curse first, because Tzadikim first suffer, and enjoy tranquility at the end; whereas for Resha'im, the order is reversed. But HaSh-m blessed Avraham, and began with "va'Avarechah Mevarechecha" (12:3)!
Ramban: HaSh-m concluded with a Berachah, "v'Nivrechu Vecha;" He thus began and ended with a Berachah.
Refer to 27:29:5:5.
Gur Aryeh: That Pasuk, at the beginning of Lech Lecha, was not a direct blessing, but an assurance from HaSh-m that He would do so in the future. The order is only significant at the time the blessing is pronounced.
Rashi writes that Yitzchak put the curse first, because Tzadikim first suffer, and enjoy tranquility at the end. HaSh-m commanded (Devarim 27: 12,13) to give Berachos on Har Gerizim, before the Kelalos on Har Eival!