Where did the northern border of Binyamin begin?
Rashi: It started at the Yarden, even with Kesef (north of) Yericho, and went to the west.
What is the meaning of "Yamah"?
Bi'ur ha'Gra: Towards a sea on top of the mountain.
Malbim: It is westward.
What is "Midbarah Beis Aven"?
Rashi: It is to the Midbar of Beis Aven.
What is the meaning of "v'Hayu Totz'osav Midbarah Beis Aven"?
Bi'ur ha'Gra: The border bent towards the Midbar of Beis Aven.
Is Beis Aven the same as Beis Kel?
Bi'ur ha'Gra: It is not the same 1 , for Beis Kel is south [of the border between Binyamin and Efrayim], and Beis Aven is north of the border. It says here (verse 13) that the border went straight from Beis Aven to Luz, and regarding Efrayim's border (north of Binyamin), it says va'Yatza (it bent) from Beis Kel to Luz. 2 I.e. the border bent from north to south, and Efrayim's boundary widened.
Hagahah: One might have thought that they are the same due to their place on the border. (Yosef's border went from Yericho to Beis Kel to Luz to Ataros (16:1, 2), and Binyamin's goes from Kesef Yericho to Midbar Beis Aven to Luz to Ataros. Really, one could say that Beis Aven is Beis Kel, if Midbar Beis Aven is south of the city.
Above (7:2), it says that Ai with (near) Beis Aven was east of Beis Kel. Here it says that the mountain, i.e. of Beis Kel, was east of Beis Aven!
Malbim: Even though Beis Aven itself was east of Beis Kel, Midbar Beis Aven extended west of Beis Kel; here it says that Midbar Beis Aven was to the west.