Who are the Zekenim, and why does it say that all Ziknei Yisrael came? Verse 1 said that all Shivtei Yisrael came!
Radak (Shmuel II, 5:3): Our verse means that the Chachamim already came first to David to make a Bris, lest he punish them for supporting Ish Boshes for two years, and delaying five years before coming to make David king. After the Bris he sent them [to bring] Shivtei Yisrael, and they came and anointed him.
Malbim: They are the Sanhedrin. They must authorize appointment of a king.
Why did David make a Bris with them?
'Rashi': This is like the Bris that Yehoyada made "between Hashem and the king and the nation" (Melachim II, 11:17) ? they will be servants to Hashem, and also between the king and the nation ? they will be servants, according to the laws of kingship. Also the king must do like his law, to fight their wars!
Why does it say "Lifnei Hashem"? The Mishkan was not in Chevron!
'Rashi': Wherever they make a stipulation of a Bris, He is there. We find regarding Yiftach "Lifnei Hashem ba'Mitzpah" (Shoftim 11:11), and "u'Ma'alah Ma'al ba'Shem 1 v'Chichesh ba'Amiso" (Vayikra 5:21).
Radak (Shmuel II, 5:3) #1: The Shechinah dwells wherever all or most of Yisrael are.
Radak (Shmuel II, 5:3) #2: They said that Hashem should be a witness between us in this Bris that we make.
I.e. denying a debt to one's colleague when there are no witnesses is called Me'ilah ba'Shem, for He is the third party involved (PF, based on Rashi Vayikra 5:21).
Why did they anoint David? Shmuel already anointed him (Shmuel I, 16:13), and also Yehudah did (2:4)!
Radak (Shmuel II, 5:3) Here it says that they anointed him to be king over Yisrael, like Hashem said through Shmuel. Shmuel anointed him over all of Yisrael, just like he anointed Sha'ul, but it was not fulfilled until today. This is why David fought the Yevusi in Tziyon only after this.
