1)
What is the meaning of "va'Yifretzu"?
1.
Radak: It is like va'Yiftzeru (they insisted), like "va'Yiftzar Bam Me'od" (Bereishis 19:3).
2)
Why does it say "El ha'Mitah"?
1.
Radak: This is like Al ha'Mitah, just like "El he'Harim Lo Achal" (Yechezkel 18:6) is like Al he'Harim, and many others.