1)
There was no king in Edom, rather, a Netziv (Melachim I, 22:48)!
1.
Malbim: "Melech" refers to the Netziv (a Yisrael appointed to rule over them).
2)
Why does it say "va'Yasobu"?
1.
Malbim: The straight path was along the border of Eretz Yisrael. They went around Har Se'ir via the Midbar, just like Yisrael did when coming to Eretz Yisrael. There was no water there in the days of Moshe, and also now.
