1)
Why does he command them to repent now?
1.
Radak: In any case they will repent when they see Hashem's great act against Machaneh Ashur; they will get a greater reward if they repent beforehand, and believe the Navi's words.
2.
Malbim: If you repent, you will be saved. All the evil came to you only because you veered from Him!
2)
What is the meaning of "He'emiku Sarah"?
1.
Rashi: Those who plotted deeply how to veer from Him, and did so.
