Why is there a tradition to write "va'Yomer" with a Vav at the end, but it is not pronounced?
Radak: With a Vav, it means Yehoshua and the Kohanim. Without a Vav, it means Yehoshua alone.
What is the meaning of "Ivru v'Sobu"? Where do they pass?
Alshich: Hashem intended that only the Chalutz will go in front of the Aron. Refer to 6:8:1:1.
Malbim: Always, the Aron is in front of the people. Here, the people are in front of it. Verse 4 says so (about the Kohanim). Before, the Chalutz (Bnei Gad and Bnei Reuven) went in front of Bnei Yisrael (4:12). Now, the order is reversed; what is more important is closer to the end. The Aron is last, the Chalutz in front of it, and the rest of the nation in front. The stipulation (Bamidbar 32:21) was that Bnei Gad and Bnei Reuven will go in front of Hashem. Since this victory was miraculous, the Chalutz went in front of the Aron, and not in front of Yisrael. The rest of the war will be close to natural, so the Chalutz will be in front of Yisrael to fight and conquer. 1
Malbim: It would not be honorable for the Aron if the nation stands still, and the Chalutz will go in back and the Aron in back of them. Rather, the Aron and Chalutz stood still, the nation passed in front, and then the Chalutz passed in front of the Aron. This order showed that the victory will be via the Aron. The Chalutz is next to it, just like the Kreisi u'Pleisi (archers and slingshot shooters) are next to the chief general.