Which angel said this?
Rashi: It is "ha'Dover Bi" (9). Radak ? this is the preferred answer. Alternatively, it is the angel standing amidst the Hadasim to help Yisrael.
Why does it say "Ad Masai Atah Lo Serachem Es Yerushalayim"?
Radak: As long as the Nochrim are serene, Yisrael will not leave Galus, until Malchus Bavel ceases. This was a vision of what already happened. He saw that Bavel's serenity and destruction were all from Hashem, via conducting angels. They strove for the good of Yisrael. Also the reign of the other three kingdoms and their fall, all was from Hashem, just like he showed him regarding Malchus Bavel.
Malbim: The angel saw that a night [Galus] ends, and another night comes ? another kingdom seizes authority over Yisrael. They will do great evil to Yisrael, e.g. in the days of Antiyochas, who pained Yisrael greatly. He saw that this is still not the time of Ge'ulah. Therefore, "va'Ya'an" (he spoke in a loud voice). He stressed, why do You not have mercy ? what good is the mercy of the horses that go in the land? Why don't You Yourself have mercy on Yerushalayim?
What were the 70 years of anger?
Radak #1: If this refers to the past, these were the 70 years of Malchus Bavel. There was anger at the entire kingship of Nebuchadnetzar.
Radak #2: If this refers to the 70 years until the vision, which was in Daryavesh's second year; it completed 70 years from the Churban. Even though this vision was in Shevat, and they already started building in Kislev, there was still anger, and the entire time of the building, there was war with their tormentors.
They are the 70 years of Galus decreed due to the years that Yisrael did not fulfill Shemitah and Yovel. It seems that Malbim explains so, for he says, since You were already angry for 70 years, the time for Ge'ulah already came! (PF)
