What is the meaning of "va'Yatziku"?
Rashi: This is like va'Yatzigu 1 ; they set it up 2 on one side, and the people passed on the other side. David wanted to carry it with him.
Malbim: They made for the Aron a Matzuk (high, fortified) place to put it. This is like "Metzukei Eretz", "ha'Shen ha'Echad Matzuk mi'Tzafon (1:2:8, 1:14:5).
Radak: Or va'Ya'amidu. The same applies to "va'Yatzikum Lifnei Hashem" (Yehoshua 7:23).
Malbim (27): This was so the Kohen could stand in front of it when asking from the Urim v'Tumim.
Why does it say "Evyasar ascended"?
Rashi citing Yuma 73b: He asked from the Urim v'Tumim, and was not answered; he was removed from being Kohen Gadol, and Tzadok replaced him. Radak - this is because Tzadok asked and was answered.
Malbim: He ascended the high place on which they put the Aron and stood in front of it to guard it, for its honor.
Why does it say "Evyasar ascended until the people finished passing"?
Rashi: The verse should be re-arranged. They set up the Aron until all the people finished passing, and Evyasar ascended.
Malbim: He ascended and stood in front of the Aron until everyone passed. According to Chazal, this is because he asked from the Urim v'Tumim, and was not answered.
Rashi (citing Yuma 73b) writes Evyasar asked from the Urim v'Tumim, and was not answered. What is the reason?
Radak: Evyasar was from Beis Eli. The curse on them was now fulfilled. Tzadok was from Pinchas, who was promised "Lo ul'Zar'o Acharav Bris Kehunas Olam" (Bamidbar 25:13). However, Tzadok was not anointed or appointed Kohen Gadol until Shlomo became king - "va'Yimshechu la'Shem l'Nagid ul'Tzadok l'Chohen" (Divrei ha'Yamim I, 29:22). The curse was suspended until Shlomo became king - "Shlomo expelled Evyasar from being Kohen" (Melachim I, 2:27). "V'Hibateta Tzar Ma'on 1 " (Shmuel I, 2:32) - Ma'on is the Beis ha'Mikdash, which was not built until the days of Shlomo.
Rashi (Shmuel I, 2:32): You will see your competitor (who will receive what you lost) next to you in My house, like a woman who sees her co-wife with her in the house.
