1)
Why does it say "va'Yashuvu... Lashuv... b'Simchah"?
1.
Malbim: They resolved to return in Simchah, like the coming verse teaches that they took musical instruments.
2)
How did Hashem make them rejoice from their enemies?
1.
Malbim: They saw that Hashem caused the enemies to attack, in order to make them rejoice over His salvation of them.
