1)
Did he gather everyone?
1.
Malbim: He gathered "Kol Bachur" (Shmuel II, 6:1), i.e. their chosen ones ? the judges from every city. It is chosen. They were the Chachamim.
2)
Why does it say "from Shichor Mitzrayim until l'Vo Chamas"?
1.
'Rashi', Vilna Gaon, Malbim: This is like "mi'Levo Chamas Ad Nachal Mitzrayim" (Melachim I, 8:65; from the north of Eretz Yisrael to the south).
