Why does it say "v'Lo Yuchlu
Malbim: They cannot stand [against the enemy] naturally, and not via Hashgachah. Cherem curses their natural strength and weakens. Hashgachah will not help them, for "I will not be among you if you do not eradicate the Cherem."
Why does this verse say "Tashmidu ha'Cherem", and the next verse says "Hasirchem ha'Cherem mi'Kirbechem"?
Malbim: If the Cherem is not among them, i.e. they burn it, their natural strength will return. Hashgachah will return only if they eradicate the one who transgressed the Cherem.
Refer to 7:13:2:1.