1)
Why will the Tzadikim rejoice?
1.
Radak: They will truly rejoice in Hashem, for they will see His straight judgments.
2.
Malbim: It is because Hashem clings to them with love.
2)
What is "l'Zecher Kodsho"?
1.
Radak: It is His name.
2.
Malbim: Do not thank for natural conduct, rather, for His wonders in a Kodesh way, above nature.