1)
What is the meaning of "[Mal'ach Hashem] b'Mal'achos Hashem"?
1.
Rashi: It is a mission from Hashem. Radak - "Mal'ach" usually means a Shali'ach. "Va'Yishlach Mal'ach va'Yotzi'enu mi'Egypt" (Bamidbar 20:16) refers to Moshe.
2)
Why did He say "Ani Itechem"?
1.
Malbim: Do not fear, and do not wait for the king to authorize rebuilding the Bayis. I am with you to save you - the king will not harm you.
