1)
Why is he called Mal'ach Hashem? Verse 20 called him Mal'ach Elokim!
1.
Malbim: Only Hashem can accept a Korban. The dialogue and command are with Mal'ach Elokim, but when he touched the stick to the Korban to accept it, it had to be Hashem Himself dwelling in him, so he is called Mal'ach Hashem.
