Who were "the Chalutz"?
Rashi: They are Bnei Gad and Bnei Reuven. Bnei Gad were mighty, and struck with a mighty arm (and cut off the enemy's arms with their heads)
Were the Chalutz in front of all the Kohanim?
Malbim: No, they were in front of only the seven Kohanim "Tok'ei (who were blowing) ha'Shofaros." The rest of the Kohanim were with Shevet Levi, in front of the Chalutz.
Why is "Tok'ei" written with a Vav at the end?
Radak: It can be read like 'Tak'u' (past tense, they blew).
Who were "the Me'asef"?
Rashi (13) Radak, citing the Targum: It is Shevet Dan, "Me'asef l'Chol ha'Machanos" (Bamidbar 10:25). Rashi
Were the Me'asef blowing the Shofaros?
Radak, citing the Targum: No, the Kohanim were blowing the Shofaros.
Why does it say "Haloch v'Sako'a"?
Malbim: They did not cease blowing while walking. Verse 8 taught only that they blew while the people passed in front.