What are "Kohanei ha'Mishneh"?
Rashi: They are Signei Kehunah. They 1 are second to the Kohen Gadol. Metzudas David - this includes also the Mashu'ach Milchamah (who speaks to the soldiers when they go to fight).
Metzudas David: It is the Segan and the Mashu'ach Milchamah
What are "Shomerei ha'Saf"?
Rashi: They are appointed over all needs of the Mikdash. They have the keys of the Azarah.
Refer to 22:4:3:1-2.
Why did he eradicate idolatry?
Malbim: There (in Parshas Nitzavim, shortly after the Tochachah), it warns against idolatry ("Pen Yesh
Why did they need to remove the idolatry from the Heichal? Menasheh did so when he repented - "va'Yasar Es Elohei ha'Nechar v'Es ha'Semel mi'Beis Hashem" (Divrei ha'Yamim II, 33:15)!
Radak: Amon returned it. Yoshiyah began to remove the idols in his 12th year of kingship, and finished in his 18th year, after they found the Sefer Torah.
According to the opinion that Yoshiyah sinned and repented (Shabbos 56b), perhaps he himself had returned it! (PF)
What is "Shadmos Kidron"?
Rashi, Targum Yonasan: It is the plain of Kidron.
Why did he take the ashes to Beis Kel?
Rashi: It is a Tamei place. There he was Metamei 1 the Mizbe'ach that Yaravam made, and burned Kohanei ha'Bamos on it. Radak - it was the primary place of idolatry. Yaravam's calf was there 2 .
Radak (verse 8): "Va'Ytamei" means that he made it a place of Tum'ah, to cast there Neveilos and all Tum'ah, to distance idolatry from the hearts of the nation.
