1)
What does this Mizmor discuss?
1.
Radak: Also this discusses the gathering of exiles.
2.
Malbim: It discusses the famine of three years in the days of David. "Va'Yomer Hashem El Sha'ul v'El Beis ha'Damim [Asher Hemis Es ha'Giv'onim]" (Shmuel II, 21:1), and they gave seven on Sha'ul's descendants to the Giv'onim, to kill them. Afterwards, Hashem was appeased. Chazal have a tradition that many converted then, for they saw how Hashem judges [those who afflict] converts, and they feared Hashem and His straight judgments.