Why did he say "Yesh Oso Devar Hashem"?
Radak: He did not come to fight! Why did he come? Surely it was via Nevu'ah (Hashem told him). He has Hashem's word [for us]! Malbim - "Oso" is in place of "Ito" (Hashem's word is with him). It says "Oso", to show that since Oso (he is found here), surely there is Ito. He would not come to the army camp had Hashem not sent him!
Why does it say "va'Yerdu"?
Radak: They descended from their chariots for his honor, to speak to him. Because they were in great distress, they were concerned for the honor of the Creator and His prophet. Their reward for this was meriting to see the following miracles.
Why is Yehoshafat not called Melech?
Radak: Because he spoke (and is mentioned by name earlier in the verse, also now) he is mentioned by his name. "Melech" Edom is the Netziv, who was like a king for them.
Radak citing Bamidbar Rabah 21:6 #1, Malbim: This shows that Tzadik's humility. He did not want to descend in front of the Navi in royal clothing, rather, like a commoner.
Radak citing Bamidbar Rabah 21:6 #2: There was a decree that he be killed with Achav [in the war of Ramos Gil'ad]. Kingship is counted to his son from then and onwards. Therefore, he is not called king.
