1)
From which exile does the verse discuss gathering them?
1.
Radak: This can refer to Galus Bavel. However, based on the coming Parshah, it seems that it refers to the final Galus.
2.
Malbim: It is from [Bavel], to which they scattered them.
2)
Why did Hashem promise "u'Faru v'Ravu"?
1.
Malbim: They will multiply, to counter "Lo Fekadtem" (verse 3 - you did not prevent loss of them).
