1)
Why does it say va'Yavo (singular)?
1.
Rashi: They all had one intent - for evil.
2.
Rashi citing Shir ha'Shirim Rabah 8:7: This hints to Hashem ? he told the Navi beforehand who will come to him [Radak ? with evil intent. They portrayed themselves as if they want to seek Hashem].
2)
What is the significance of "Ziknei Yisrael"?
1.
Rashi: They were Resha'im. "Ziknei Yehudah" (8:1) were Tzadikim.
2.
Malbim (5) #1: Refer to 14:5:1:4.
3.
Malbim (5) #2: They were false Nevi'im. They came to hear the Navi's words, and switch them according to their desires ? "Meganevei Devarai Ish me'Es Re'ehu" (Yirmeyah 23:30).