Even though the idea of preaching to a major Lamanite king, particularly one in the land of Nephi, would have been a very attractive proposition to Ammon, the Lord knew that there were other important tasks. The Lord directs Ammons efforts to the recovery of his brethren in Middoni.
Aaron is the brother of Ammon, but Muloki and Ammah are simply fellow travelers (the sons of Mosiah are Ammon, Aaron, Omner, and Himni, see Mosiah 27:34). We have no indication of why Ammon chose to travel alone, while Aaron took two companions with him.