What is the meaning of "Im Beis Aven"?
Why does the Navi mention the cities near Ai?
Radak: This is to specify which Ai. There was another city Ai among the cities of Amon 1 , mentioned in Yirmeyah (49:3) "Shudedah Ai." This one was bigger and known, therefore it is always mentioned with the prefix Hei.
Malbim: This shows how they suffered a 'chance' mishap [after the Hashgachah departed due to Achan's sin]. Yehoshua sent spies to investigate "ha'Aretz", i.e. Ai and its surroundings. They investigated only Ai, and said that it is a small city and a small amount of men will suffice. Really, there were great cities nearby that would come to help it (we find that everyone from Beis Kel came to help it the second time
Obviously this is not the Ai of Amon! All of Amon (including what Yisrael took from Sichon) is east of the Yarden, and now Yisrael were on the west! (PF)
Why does the Navi again call the spies "men"?
Malbim: This shows that they acted on their own (and not as Yehoshua's messengers), and spied out only Ai (refer to 7:2:2:2).