1)
Who was the Navi?
1.
Radak citing Seder Olam 20: It was Amotz, the father of Yeshayah.
2)
Why did he say "Ein Hashem Im Yisrael"?
1.
Malbim: If you rely on Hashem, know that he is not with Bnei Efrayim. We announce in war that one who fears due to his Aveiros should return home!
3)
Why is Yisrael attributed to Efrayim?
1.
'Rashi': Yaravam ben Nevat, from Shevet Efrayim, was the first to cause Yisrael to sin. His golden calves were a stumbling block for every generation after him; they served them.
