1)
What is "Kol Davarav"?
1.
Rashi: It is the stipulation between them.
2.
Malbim: He stipulated with them about the extent of his authority, just like Shmuel told the people the laws of the king before they anointed Sha'ul.
2)
Why does it say "Lifnei Hashem ba'Mitzpah"?
1.
Rashi: This is where they all gathered (10:17). The Shechinah dwells on a big Tzibur.
