Why is there no Vov before "Tapchem"?
Radak: There are verses in which it is omitted, e.g. "Shemesh Yare'ach" (Chabakuk 3:11), "Reuven Shimon" (Shemos 1:2). 1
When verses list many things, often there is a Vov before each name after the first (e.g. Yefes' sons, Yaktan's sons, cities in Ever ha'Yarden, and the Tamei birds (Bereishis 10:3, 26-29, Bamidbar 32:3, Devarim 14:12). Sometimes a minority do not have a Vov, e.g. the 66 who descended to Egypt, the Kosher animals, and cities given to Shevatim (Bereishis 46:9-24, Devarim 14:4,5, Yehoshua 15:20-62). (PF)
What is Chamushim?
Radak: It is like Chalutzim (Bamidbar 32:30) 1 .
Tosfos (Kidushin 61b DH Iy): I.e. it is at the front of the army. Refer also to Bamidbar 31:3:2:1-4 and the note there.
Were only the Giburei Chayil (valiant soldiers) obligated to fight?
Malbim: Yes! Since you will go "Lifnei Acheichem", and not Lifnei Hashem, i.e. it will be a natural war, only the valiant men will go 1 . If it were miraculous, everyone would go!
Only about 40,000 men came to fight from Reuven, Gad and part of Menasheh (below, 4:12-13). Just from Reuven and Gad, there were over 84,000 men at a proper age to fight (Bamidbar 26:7, 18)! The exemptions due to marriage, vineyards, houses and fear do not apply to the war to conquer Eretz Yisrael (Sotah 44b).
