Why do verses 6-8 all say "v'Alah ha'Gevul", and verse 10 says "v'Yarad"?
Rashi: Going from the east towards Yerushalayim, [the land] ascends (and past Yerushalayim it descends
Was Beis Chaglah within the border?
Malbim: It was outside Yehudah's border, and within Binyamin's (18:21).
Here it says that that the border was north of Beis ha'Aravah, i.e. the city was in Yehudah, and so it says below (verse 18). The city is listed in Binyamin's portion (18:22)!
Malbim: There were two cities called Beis ha'Aravah
What is "Even Bohan Ben Reuven"?
Radak: The local ruler was named Bohan Ben Reuven (Targum Yonasan). The place was named Even. Perhaps there was a big rock to denote the border.