Rashi (verse 3) writes that going west towards Yerushalayim, the land ascends, and past Yerushalayim it descends. Here it ascends even after Yerushalayim!
Rashi: Ein Eitam is slightly higher than Yerushalayim. David thought to build the Beis ha'Mikdash there (Zevachim 54b).
Why is Yerushalayim called Kesef ha'Yevusi?
Malbim: Yevus lived there.
Was Yerushalayim in Yehudah's portion?
Rashi: The border was south of Yerushalayim; it was in Binyamin's portion. 1
Yuma 12a #1: Har ha'Bayis, and the chambers and Azaros [of the Mikdash] were in Yehudah's portion, and also the Mizbe'ach was built on a strip that jutted out from Yehudah's portion into Binyamin's. The Ulam and Heichal were in Binyamin's portion.
Yuma 12a #2: Yerushalayim was not given to any Shevet.
What is Rosh ha'Har?
Malbim: It is the beginning of the mountain at the end of its slope. In Binyamin's portion, it is called (18:16) it is called Ketzei ha'Har.
What is Gei Ben Hinom?
Radak citing Targum Yonasan: It is the valley 1 of Ben Hinom, close to Yerushalayim. Yoshiyah declared it Tamei due to the idolatry there. The previous ruler there was Ben Hinom.
Radak: If it was a valley, why does it say v'Alah ha'Gevul? It does not say so regarding the valley, rather, the place in which the valley was (i.e. around it); Yerushalayim was higher than all lands.
Where was the mountain?
Malbim: It was establish of Gei Ben Hinom, and north of Emek Refa'im.
Here, the verse lists [on the border] Gei Ben Hinom, Kesef Yevusi, ha'Har. Below (18:16), the order is different - ha'Har, Gei Ben Hinom, Kesef Yevusi!
Bi'ur ha'Gra: Yehudah's border is listed from east to west. Below, Binyamin's is listed from west to east. Kesef Yevusi is not a place on the border; it merely clarifies where is Gei Ben Hinom.
What is Emek Refa'im?
Radak, citing Targum Yonasan: It is the plain of the mighty ones. It used to be a place of giants.