Why did Moshe gather everyone?
Rashi #1 and Targum Yonasan: He gathered them together before Hakadosh-Baruch-Hu (so that everyone should be aware of the B'ris and be included in it) 1 in order to bring them into the Bris. 2
Rashi #2 (in Pasuk 12): Since Yisrael were about to go from one leader to another, he stood them together as a group in order to stimulate them. 3
Da'as Zekenim (in Pasuk 10) and Rosh: He hinted that they can stand and endure only when they are one group.
See Targum Yonasan.
See end of Ki Savo. Da'as Zekenim and Rosh (both in Pasuk 11) and Hadar Zekenim. Now, no-one can say that he was not there, and that he did not therefore accept the Bris' - And once they cross the Yarden and go to their respective inheritances, it will be impossible to gather everyone
What are the implications of "Atem Nitzavim ha'Yom ... "?
Rashi (in Pasuk 12, citing the Midrash Tanchuma): The Torah juxtaposes "Atem Nitzavim" to the curses, in order to reassure them. After they heard the ninety-eight curses, besides the forty-nine curses in Bechukosai, they turned green from shock, and exclaimed 'Who can withstand all of these?' So Moshe put their minds at ease - 'So many times you angered Hashem, yet He did not annihilate you; See how you are still standing before him!' 1
Lev Eliyahu p.263: Moshe did not mean that the curses are meaningless, but that, since they will take them to heart, there will be no need for them to be fulfilled
What is the meaning of "Rosheichem Shivteichem"?
Rashi: There is a 'Lamed' missing in front of "Shivteichem", which therefore translates as 'the leaders of your tribes'. 1
Hadar Zekenim and Targum Onkelos: They refer to the leaders (the Nesi'im), and the tribes themselves.
Targum Yonasan: They refer to the heads of the Sanhedrin and the leaders of the tribes (the Nesi'im).
Yerushalmi Horayos, 3:5: Rosheichem Shivteichem" refers to the lay leaders of the tribes; Zikneichem, to the Talmidei-Chachamim.
See Sifsei Chachamim.
Why did Moshe give precedence to the Rashim over the Zekeinim, whereas Yehoshua - in 24:1 - reversed the order?
Yerushalmi Horayos, 3:5: Refer to previous answer. There are four reasons for this: Because 1. ... whereas they were all (even the Rashim) Moshe's Talmidim, the Rashim were not Talmidim of Yehoshua - and it would have been an insult to the Zekeinim to give the Rashim precedence; 2. ... whereas Moshe did not need the Zekeinim to help him in issuing Torah rulings, Yehoshua needed their advice in conquering the land; ; 3. ... whereas Moshe did not become fatigued when he studied Torah, Yehoshua did - in which case he constantly needed the Zekeinim's support; 4. ... Moshe saw with Ru'ach ha'Kodesh, that Yisrael were destined to go into Galus and that they would not manage to survive without the leadership of the Rashim. 1
See Torah Temimah, note 7.
What is the significance of the order in which the various groups are listed?
Rashi: They are listed in decreasing order of importance.
Why did Moshe need to mention the different groups? Why did he not simply say "Kol Ish Yisrael ve'Gercha"?
