1)
Will they then know that He is Hashem?
1.
Malbim: They will know that I give reward and punishment. Via this I will gain from punishment ? via them they will repent and know Hashem.
2)
Why does it say "El Chinam"?
1.
Radak: This is like b'Chinam. What I said will be fulfilled. Also "v'El ha'Aron Titen Es ha'Edus" (Shemos 25:21) is like uva'Aron.
2.
Malbim: I did not say to do this evil for no reason. Via this decree, I will attain My desired end.
