What is the Chidush "Ach Ami Hemah"?
Rashi: Even though it is revealed in front of Me that they will betray Me, in any case they are My nation; they are in front of Me like "children who will not lie."
Radak: Hashem said so when He took them out of Egypt. I took them from amidst another nation, and took them for children to Me; "they will not be false to Me." "Ach" shows the truth of the matter, like "Ach Atzmi u'Vesari Atah" (Bereishis 29:14), "Ach Tov l'Yisrael Elokim" (Tehilim 73:1).
Malbim: This is why it is proper that I have mercy on their affliction.
Why does it say "Banim Lo Yeshakeru"?
Malbim: Banim is a higher level than Ami. Therefore, they are worthy of great Chasadim, to do wonders for them, like a father does for his sons. Since they will not be false, they will thank Me for the good.
Why does it say "va'Yhi Lahem l'Moshi'a"?
Radak: Since He saved them from Egypt, "they will not [ever] be false to Him."
Malbim: Since they will not be false, and will thank Me for the good, I will always save them.
