With whom did David consult before speaking to all the assembly of Israel?

He consulted with the commanders and with every leader in Israel. [13:1; 13:2]

Why did the assembly agree to listen to David, send out messengers everywhere in Israel and join with David to  bring the ark of God back to Israel?

The assembly agreed to do these things because they seemed right in the eyes of all the people. [13:2; 13:4]