What is the meaning of "Kum Lach"?
Rashi #1: "Kum" is written without a Vov. We read this like "Kam Lach"
Moshe said this! Since it is written in the Torah, it is as if Hashem said it. Or, Hashem chose Yehoshua to be leader in response to Moshe's request. (PF)
What is the meaning of "Lamah Zeh Atah Nofel Al Panecha"?
Radak: This is like "Mah Titz'ak Elai" (Shemos 14:15). You do not need supplication. Eradicate the Cherem, and I will forgive them!
QUESTIONS ON RASHI
Rashi writes that Yehoshua was at fault for making the Cherem. He followed his Rebbi! Moshe made Cherem three times (refer to 7:20:4:1-3 and the notes there)!
Me'iri (Sanhedrin 44a): Moshe made Charamim before Yisrael crossed the Yarden, and they were not responsible for others' sins. Yehoshua's Cherem was after they crossed, and were responsible for others' sins. 1
Maharatz Chayos (44a, according to Rav): Yehoshua chose to make the Cherem. Hashem commanded Moshe to make Cherem all three times.
Me'iri: According to R. Yehudah, they were responsible for others' hidden sins only after crossing. According to R. Nechemyah, they were responsible for others' open sins only after crossing.