1)
Why does it say "Ad Ketzei ha'Yarden"?
1.
Rashi: The width (from south to north) of Yehudah was only opposite Yam ha'Melach, i.e. until the point where the Yarden enters it.
2)
Where is the northern border of Yehudah?
1.
Rashi: It begins [in the east] where the Yarden enters Yam ha'Melach, and goes west to Beis Chaglah?, like verse 6 says.
