1)
What will remain?
1.
Radak, Malbim: Not all of you will fall to the sword.
2)
What is the grammatical form of "b'Hizaroseichem"?
1.
Radak: It is Makor (like an infinitive) with a Yud to indicate plural, because [without the Yud] it is similar to a noun. The same applies to "bi'Vnosayich Gabech" (16:31).