1)
Did Yisrael not come to fight David's men when they approached?
1.
Malbim: No, because there was fear in their hearts. They stood close to the Yarden.
2)
How could the war be in Ya'ar Efrayim? Ever ha'Yarden was only for Gad, Reuven and Chetzi Menasheh!
1.
Rashi, from Bava Kama 81a: Yehoshua made a condition (for the inheritance of Eretz Yisrael) that people may graze animals in forests (Radak - of other Shevatim). This forest was close to Efrayim, just the Yarden interrupted. [Bnei Efrayim] grazed their animals there, and it was called Ya'ar Efrayim.
