1)
Why did they decree not to give their daughters to Binyamin?
1.
Radak: They gave to them the law of Menudim (excommunicated), because they did not give the sinners [to be punished]; people may not marry into them.
2)
Why did they regret (verse 7) the Shevu'ah not to give their daughters to Binyamin so soon after swearing?
1.
Malbim: Really, they swore at Mitzpah before the war. Whenever the subject precedes the verb, the verse is out of order.