What is the significance of the juxtaposition of the Mitzvah of Teki'as Shofar to the Korban Musaf ?
Yerushalmi Rosh Hashanah, 4:5: It serves as a proof that the Shofar should be blown during Musaf? 1
See Torah Temimah, note 6, who cites the Bavli and elborates.
Why does the Torah insert the word "va'Asisem Olah" regarding the Olas Rosh Hashanah, instead of 've'Hikravtem Olah'?
Moshav Zekenim #1, Hadar Zekenim and Da'as Zekenim (on 28:25): Because it is the day of judgment, and Hashem says 'From when you enter in front of Me for judgment and come out be'Shalom, I consider you like a new creation!' 1
Moshav Zekenim #2: It is a hint that the world was created in Tishrei.
Ateres Zekenim 581:1, Eliyahu Rabah 581:8: To teach us 2 to make ourselves like a Korban on Rosh Hashanah. 3
See Torah Temimah, note 12.
It seems that this is what the Riva (28:19) means, only there are words missing from the text.
Ateres Zekenim and Eliyahu Rabah (Ibid.): Just as one checks a Korban for blemishes four days prior to bringing it on the Mizbe'ach, so too do Ashkenazim begin reciting Selichos at least four days before Rosh Hashanah to check [for Aveiros].