1)
Why does it say "v'Yashvu va'Midbar la'Vetach"?
1.
Malbim: [Initially] they will fear to leave flocks in the Midbar during the day due to plunderers. I will make Bris Shalom, and enemies and plunderers will not pass through [the land], and they will dwell confidently in the Midbar.
2)
What is the meaning of "v'Yashnu ba'Y'arim"?
1.
Rashi citing Targum Yonasan: They will become old in forests.
2.
Rashi: They will sleep in forests. Radak, Malbim ? they will not fear Chayos, for I will cut them off.