Why will the angel not forgive those who do not obey him?
Rashi: a. Because he is not accustomed to sin, since he belongs that do not sin 1 , and b. because, as a Shali'ach of Hashem, he is obligated to carry out Hashem's command, in which case he has no mandate to pardon those who sin. 2
Rashbam: Because He speaks in Hashem's Name. 3
Da'as Zekenim: Because Hashem's Name is contained in his, and rebelling against him is tantamount to rebellion against Hashem. It is therefore only Hashem who can pardon the sinners.
Moshav Zekenim: He does not understand what sin is [so he cannot pardon it].
Since only Hashem Himself has the power to pardon, but not His emissaries, as the Pasuk says in Tehilim, 130:4 "because forgiveness is with You" (Rashi there).
Why does the Torah switch from the singular ("Al Tamer bo") to the plural ("Ki Lo Yisa le'Fish'achem")?
Seforno: To teach us that, even when one person sins, the entire community suffers 1
As we find by Achan - in Yehoshua, 22:20. (Seforno).
What are the connotations of "Ki Sh'mi be'Kirbo"?
Rashi #1 and Ramban #1: It is because Hashem's Name is contained within the angel that Hashem will send, that one should take care not to disobey him. 1
Rashi #2: This refers to the angel Matatron, whose Gematriya (three hundred and fourteen) is equal to that of Hashem's Name, Shakai.
Ramban #2 (citing Kabalah): This is the Redeeming Angel, 2 which, according the Gemara in Sanhedrin, 38b) is synonymous with Matatron referred to in Vay'chi, and whom he discussed at length in Bereishis 46:1. 3
Ramban #3, Rashbam, Targum Onkelos and Targum Yonasan: Do not disobey him because he speaks in My Name. 4
Hadar Zekenim: Hashem's name is a part of the name of every angel - whose names end with the letters 'Keil'.
Rosh: Sh'mi is an acronym for Sonei Matanos Yichyeh. Do not think that you will sin and give to him bribes to pardon you!
Moshav Zekenim: He has a form of tablets opposite his heart, and Hashem's name is written on them. 5
And the one who disobeys him therefore deserves to be destroyed via the Midas ha'Din (Ramban).
See Ramban here and refer to, 12:12:7:1*.
Which is synonymous with the Shechinah (Kevayachol), and whom Moshe replaced during his lifetime (See Sh'mos 7:11), and who appeared to Yehoshua prior to the conquest of Cana'an. The Ramban goes on to name him as the angel Gavriel. In any case, this is not the angel that Hashem said He would send, and Moshe succeeded in preventing it through his Tefilos (See Ramban and refer to 23:21:1:1**).
Because his voice is synonymous with the Voice of Hashem (Ramban).
Moshav Zekenim: As the Pasuk writes in Yeshayah 30:27 "Shem Hashem Ba mi'Merchak".
What was the identity of the Mal'ach that Hashem planned to send after the Chet ha'Eigel?
It was either a Navi, Micha'el, Matatron or Gavriel. Refer to 23:20:1:1*, 23:20:0.1:1*, 1 & 23:20:0.1:2-3-3 respectively.
QUESTIONS ON RASHI
Rashi writes that "Sh'mi be'Kirbo" refers to Matatron, whose Gematriya equals that of Shakai. Many names have this Gematriya?