1)
What is the meaning of "Yismach Yisrael b'Osav"?
1.
Radak: It is a glorified expression (to say that Hashem makes Yisrael), like "Aye Eloka Osai" (Iyov 35:10), "Ki Bo'alayich Osayich" (Yeshayah 54:5).
2.
Malbim: They should rejoice in Hashem, for He made them a nation - "Hu Asanu v'Lo Anachnu Amo" (100:3).
2)
Why does it specify "Bnei Tziyon"?
1.
Mesilas Yesharim (Perek 19): The more that one merits to enter more internally knowing Hashem, his Simchah will be greater.
2.
Malbim: They are the Chasidim close to Hashem. Gil is joy over a new matter. They will constantly have a new Simchah, for their King goes among them. They will attain in Him level after level.
