1)
Why does it say "Yerushas Peleitah"?
1.
Rashi: The inheritance of the Shevet is empty. Give counsel so they will father children and take possession of the remaining inheritance, and a Shevet will not be wiped out.
2.
Radak: Inheritance is from Yakov Avinu. There must be 12 Shevatim from his children forever.
3.
Malbim: Since the Shevet will be perpetuated through the 400 men with wives, we cannot permit the Shevu'ah through a Pesach of 'we never intended to exterminate a Shevet.'