Why does it mention Zera Yakov?
Radak: My Bris with Zera Yakov is that they will be an Am, [and with Zera David is that they will rule].
Radak citing Ibn Ezra: Also someone from Zera Yakov I will despise, for the Torah said that a king of Yisrael must be from Yakov, and also 1 he must be from David; these are two attributes.
Malbim: From Zera Yakov will be rulers over Bnei Yishmael v'Esav (and from Zera David will be rulers over Zera Yakov).
The latter stipulation came later, via the Nevi'im. (PF)
Why is there no mention of the Bris with Zera Aharon?
Radak (25): It was mentioned above (22) "v'Es ha'Leviyim Mesharsei Osi 1 ".
Radak citing R. Yonah the grammarian: Yakov is in place of Aharon, for two families were mentioned above. Also one of the 32 Midos of R. Eliezer Bno Shel R. Yosi ha'Gelili (Midah 20) is a matter that the verse does not apply it, rather, to a different matter. E.g. "v'Zos li'Yhudah" (Devarim 33:7). It already says "Shma Hashem Kol Yehudah"! Rather, it teaches about Shimon (it hints to a Brachah for Shimon, whom Moshe did not explicitly bless). Similarly, "Zera Yakov v'David" does not apply to Yakov, for it says Yitzchak and Yakov [later in this verse]. Rather, it applies to Aharon. A proof is, the two families mentioned 2
Radak: Even though that verse also mentioned Zera David, Zera David is repeated here, for the primary conduct of Yisrael is from fear of the kingdom.
Otzar Midrashim, 32 Midos of Agadah (Midah 20) concludes 'a proof is "from taking rulers from his seed." Who are the rulers? Kings and Kohanim Gedolim. These are from the two families mentioned, Bnei Aharon and Bnei David, that Hashem chose and despised them.' (PF)
Why does it say "El Zera Avraham"?
Radak: This is like Al. Also "El he'Harim Lo Achal" (Yechezkel 18:6) is like Al he'Harim.
Why is it written Yischak with a Sin?
Radak: It is the same root with a Sin or Tzadi (laughing). Also, Sin and Tzadi are pronounced from the same place (the teeth - PF). This is one of four places in which Yitzchak is written with a Sin.
Why is it written Ashuv, and we pronounce it Ashiv?
Radak: Both of these are the same. We find that the Kal conjugation is used for Po'el Yotzei (it acts on others) regarding this, e.g. "v'Shav Hashem Es Shevuscha" (Devarim 30:3).
The Kesiv hints that the Shechinah dwells with Yisrael during their exile, and will return with them. (PF) Megilah 29a says that this is why it says "v'Shav Hashem
Rashi (Devarim 30:3, second Perush): This is why it says "v'Shav Hashem
