1)
Why did he need to say both "Ir Nochri" and "Lo mi'Bnei Yisrael"?
1.
Malbim: Some Kena'ani cities belonged to Yisrael and paid taxes to Yisrael, and they feared Yisrael. Yevus was not subjugated to Yisrael, and did not pay taxes (1:21).
2)
Why did he say "Henah" (plural)? He discussed only one city!
1.
Radak: We will not go to this city, nor another city not of Yisrael.