1)
Why does the Torah use the expression "Le'avrecha bi'Veris"?
1.
Rashi: Because people who entered into a covenant would make a partition on both sides, and pass in between them. 1
1
Rashi: As the Navi writes in Yirmiyah, 34:18. Refer also to Bereishis, 15:10:1:1.
2)
What is the meaning of "u've'Alaso"?
1.
Yerushalmi Sotah 2:5: "Alah" is synonymous with Shevu'ah. 1 What the Pasuk therefore means is that Hashem is entering them into a B'ris via an oath.
1
See Naso Bamidbar, 5:21.
3)
Seeing as Yisrael did not swear on their own volition, why was the B'ris valid?