1)
Why does it say "va'Ya'amed"?
1.
Malbim: He enforced that they do like the Bris. He forced them to serve Hashem in his entire lifetime.
2)
Why does it say "ki'Vris Elokim Elokei Avoseihem"?
1.
'Rashi': It is like the Bris that he made with their ancestors in the Midbar, about idolatry ? "va'Tir'u Es Shikutzeihem?" (Devarim 29:16). This is why Yoshiyah removed all the idolatry!
