1)
Who speaks here?
1.
Rashi: Keneses Yisrael. Radak - the Navi speaks on behalf of Yisrael.
2)
Why will I rejoice in Hashem?
1.
Radak: This will be when the camp of Gog and Magog is eradicated. So Yonason translates, due to the miracle and redemption that You will do for Your Mashi'ach...
2.
Malbim: Hashem's strength will be aroused then.
3)
Why does it add "Agilah bEi'lokei Yish'i"?
1.
Radak: The matter is repeated in different words.
2.
Malbim: This is a new Simchah in the G-d of my salvation, i.e. His special Hashgachah over me.
