1)
Why is it written Nafotzos, and we pronounce it Nafotzes?
1.
Radak: It is written plural, regarding the Giborim and troops. It is pronounced singular, regarding the war.
2)
How did they die from the forest?
1.
Rashi citing Targum Yonasan: Wild animals in the forest killed them.
2.
Radak: They spread out, and were wounded on trees of the forest when they fled.
3.
Malbim: Trees and animals in the forest killed them.