Whom did he bring up on the wall, and why?
Rashi, based on Shevu'os 15a 1 : We require Sarim, Beis Din, Kohanim, Leviyim and two Todos to add to Kedushas Yerushalayim.
Shevu'os 16a: To add to [Kedushas] Yerushalayim, we require the king, a Navi, the great Sanhedrin and Urim v'Tumim. Rav Nachman explains that one of them suffices. Rav Huna requires all of them. Ezra could not be Mekadesh the city, for they lacked the Urim v'Tumim! He holds that Shlomo's Kedushah was permanent. Ezra made a mere commemoration of Kidush. (PF)
Did they literally go on the wall?
Rashi: No, they went outside the wall to go around the city. Malbim #2 - "Al" means near [the wall], like "v'Alav Mate Menasheh."
Malbim #1: Yes. The Ritva (Shevu'os 15b) was unsure about this. Perhaps they went on the wall in order to be Mekadesh the windows and the thickness of the wall, according to the opinion that it is [Kadosh] like inside.
What are "Shtei Sodos Gedolos"?
Rashi: They are Lachmei Todah.
Ibn Ezra: They are [Leviyim], who were stationed Lehodos (to thank) Hashem, and they said Shir Shel Todah.
Malbim: they are called Gedolos, for they are the largest of Lachmei Todah, i.e. Chametz. Each was from an Isaron 1 ; each loaf of Mitzvah was from a third of an Isaron.
A tenth of an Eifah, i.e. the volume of 43.2 eggs of fine flour. (PF)
What is the meaning of "v'Sahaluchos"?
Ibn Ezra: This is a noun, with the grammatical form of Tanchumos. They went.
