What were the "Mar'os ha'Tzov'os"?
Rashi, Ramban (citing Targum Onkelos) Seforno and Targum Yonasan: They were the copper mirrors that the women, who in a display of disdain for worldly desires, donated, which were now accepted and used to manufacture the copper basin and its stand. 1
Moshav Zekenim, Hadar Zekenim, Rosh: This does not discuss what was used to make the Kiyor, rather, its place. It was between the Mizbe'ach and Ohel Mo'ed, towards the north 2 , in order that women who go to Ohel Mo'ed will see it, and put to heart that a Sotah drinks from its water. (Da'as Zekenim - they will see Sotos drink, and be chastised.)
Which explains why later, in Pesukim 39-40, the Torah does not include the basin and the stand among the items that were manufactured from the donations of copper (Rashi and Seforno).
What is their source? In Zevachim 59a, all agree that the north must be empty of Kelim. Tana'im argue only about whether or not the Mizbe'ach may be in the north! (PF).
What does "Tzov'os" refer to?
Rashi: It is a reference to the episode in Egypt when they encouraged their husbands, who were worn out from the back-breaking work, to be intimate with them, by bringing them food and drink and enticing them via the mirrors. 1 And they subsequently gave birth to babies in the fields - who ultimately formed 'the hosts of Yisrael'. 2
What does "asher Tzav'u Pesach Ohel Mo'ed" refer to?
Rashi and Ramban #1: The women now congregated at the entrance of the Ohel Mo'ed in order to donate their mirrors.
Ramban #2 (citing Targum Onkelos), Seforno and Targum Yonasan: They congregated at the entrance of the Ohel Mo'ed to hear the word of Hashem (Seforno) or to pray (Targum Onkelos and Targum Yonasan).
Why did Moshe initially refuse to accept the mirrors?
Rashi: Because he considered mirrors, tools of the Yeitzer-ha'Ra, to be disgusting.
Moshav Zekenim #1: The opening of the Heichal was open 1 , so Yisraelim in the Azarah would be able to see [via the mirrors] the Kohen Gadol's Avodah in the Heichal, and the Torah said "v'Chol Adam Lo Yihyeh b'Ohel Mo'ed..."
Moshav Zekenim #2: The Kohanim bend down to wash from the Kiyor. It looks like they bow to their reflections.
This did not apply in the Mishkan, for there was a Masach at the opening of the Ohel Mo'ed! Perhaps we must say that Moshe objected due to the Beis ha'Mikdash. Also there, why is seeing a reflection worse than Kohanim standing in front of the Heichal and looking inside? (PF)
Rashi writes that Moshe considered mirrors, tools of the Yeitzer-ha'Ra, to be disgusting Why did Hashem tell him to accept them?
Rashi: They were precious in the eyes of Hashem, because the righteous women used them to raise the hosts of Yisrael. 1
Ramban citing Targum Onkelos, Seforno and Targum Yonasan: Because giving away their mirrors was an act of righteousness, in that they overcame the Yeitzer ha'Ra of desire.
Moshav Zekenim: A Kohen must offer a woman's Korban Lishmah, but he may not look at her face. Therefore, there was a mirror near the Kiyor, so he could see her relection. 2
Why must he see her at all? We offer Korbanos sent through Sheluchim, or bought with money placed in boxes, without seeing the owner! And what is the Isur to see a woman's face? According to Kabalah, the Kohen must see the owner's face, in order to know the severity of his sin. And due to the Kedushah of the Kohen, seeing her face could cause damage. (PF, heard from R. Meir Eliyahu.)