What is "ha'Amud"?
Rashi: It is where the king stands in the Beis ha'Mikdash. Radak - this is like it says in Divrei ha'Yamim II, 34:31 "Al Amdo."
Why does it say "b'Chol Lev uv'Chol Nefesh"?
'Rashi' (Divrei ha'Yamim II, 34:31): This is like it says there "v'Shavta
Why does it say here "b'Chol Lev uv'Chol Nefesh", and in Divrei ha'Yamim II, 34:31, "b'Chol Levavo uv'Chol Nafsho"?
Malbim (Divrei ha'Yamim II, 34:31): There Ezra clarified that Yoshiyah included himself in the Bris.
Why does it say "va'Ya'amod Kol ha'Am bi'Vris"?
Radak: They strengthened themselves in their mouths and hearts. The same applies to "ha'Ya'amod Libech Im Techezaknah Yadayich" (Yechezkel 22:14).
Radak citing Targum Yonasan: All the nation accepted to fulfill.