1)
What is the meaning of "v'Chasdi Lo Afir me'Imo"?
1.
Radak: The kingship, which is Chesed, even if he will annul My Bris and not guard My Mitzvos, I will not annul My Chesed or be false in My Emunah. There will always be a king among his sons.
2)
Why does it say "v'Lo Ashaker be'Emunasi"?
1.
Malbim: One who does not fulfill his promise, he is Meshaker in his words.
