What was the answer "Eleh Shnei Vnei ha'Yitzhar"?
Rashi #1: It is the horn of Kehunah and Malchus, which are anointed with Shemen ha'Mishchah.
Rashi #2: They are the Yetzer Tov and Yetzer ha'Ra. The latter is reversed to good in the merit of Torah.
Radak (11, 14): The angel answered concisely via a hint; the Navi understood the answer. Yitzhar is oil, like "Degen Tirosh v'Yitzhar" (Devarim 28:51). Therefore, he calls them Zeisim - Zerubavel and Yehoshua. They were anointed with Shemen ha'Mishchah - this one for kingship, and this one for Kehunah.
Why does it say "ha'Omedim Al Adon Kol ha'Aretz"?
Rashi: They request from him to return their grandeur.
Radak: They are prepared to do His will.
Radak citing Targum Yonasan: These are sons of great people who stand in front of the Master of the entire land.