1)
What is the grammatical form of "ha'Yeludim"?
1.
Radak: It is an adjective. The same applies to "ha'Yeludim ba'Midbar" (Yehoshua 5:5).
2)
What is "in this land"?
1.
Malbim: It is all the land (Eretz Yisrael). Eradication was decreed on the fathers [of Anasos] wherever they will be; on minor children it was decreed only on those born in Anasos, but not on those born in other cities. It says in Ezra (2:23) and Nechemyah (7:27) that people returned from Anasos; they were born from Anshei Anasos in other cities.