What are the connotations of "Ad Midrach Kaf Ragel"?
Rashi #1: It means that even just passing through their land without permission is prohibited.
Rashi #2 (citing a Midrash): It means that you will not get from Edom's land until the day that Hashem's Foot will tread on Har ha'Zeisim. 1
Rosh (3, citing R. Shmuel bar Nachmani): You will not get from his land until will come the one about whom it says "Darach Kochav mi'Yakov", i.e. Mashi'ach, and then "v'Alu Moshi'im
From whom did Eisav inherit Har Se'ir?
Rashi: As a reward for accompanying him down to Egypt and for remaining silent when they said that Sarah (his sister) was Avraham's sister, for which Hashem made him like his (Avraham's) son. Targum Yonasan (on Pasuk 18): On the merit of Avraham.
Why did Hashem see fit to explain to Yisrael why they were not permitted to fight Edom, Mo'av (Pasuk 9-12) and Amon (Pasuk 19-23)?
Rashbam #1: To preempt the question the people would ask in fear - why, seeing as they were about to embark upon the conquest of Cana'an, Hashem had ordered them to pass by these three lands and not conquer them.
Rashbam #2: To encourage the people, inasmuch as, when they will see how Hashem granted these nations their inheritance in honor of their (Yisrael's) forefathers, then they can be rest-assured that He will certainly grant them the inheritance that He swore to their forefathers that He would give to them.
Bava Kama 38a (regarding Mo'av): Hashem said to take vengeance against Midyan, and Moshe thought that all the more so, to fight Mo'av! 1
Da'as Zekenim (9), Hadar Zekenim (9), Rosh (9): Yisrael were promised the lands of Chivi and Refa'im. Se'ir is often called Chivi, but only because they smelled the ground like a Chivi (snake), to know what to plant. 2 People thought that Amon and Mo'av are called Eretz Refa'im, but it is not true.