Why does it say that they killed the king of Chevron? He was among the five kings that they hung beforehand!
Radak, Malbim #1: We must say that afterwards, they crowned a new king in Chevron.
Malbim #2: There were two kings of Chevron.
Below (15:13-17) it says that Kalev conquered Chevron, and Osni'el ben Kenaz conquered Devir!
Malbim #1: Yehoshua conquered the city Chevron, and Kalev conquered the open areas around it and its Chatzeros. It says below that the field of the city and its Chatzeros were given to Kalev; Chevron itself was an [Ir Miklat] of the Leviyim. The same applies to Devir.
Malbim #2: The account below really applies here. When they came to Chevron, Kalev reminded Yehoshua that Moshe promised [to give it to Kalev]. Yehoshua gave it to him; he fought with the city and killed the giants. [The conquest] was attributed to Yehoshua, for it was under his authority. Likewise, the conquest of Devir belongs here. The Navi relies on what is explained regarding the inheritances [to explain how these two cities were conquered].
What are "all its cities"?
Radak: They are the cities and the people of the cities on its border, that are called by its name.
