1)
Why does it say "Ba'avur Yishmeru
1.
Radak, Malbim: He gave the land on condition that they guard His Chukim and Toros - "Ki Evi'enu El ha'Adamah Asher Nishbati la'Avosav
1
Above (Devarim 31:16-17), it says "v'Hefer Es Brisi;
2)
Why does it say Hallelukah"?
1.
Radak: The author tells Yisrael in his generation, and their children after them, to praise Hashem for the good that He did with their fathers.