Why did Bil'am ask the emissaries of Balak to wait specifically overnight for a reply?
What did Bil'am have in mind when he said "Ka'asher Yedaber Hashem elai"?
Rashi: He thought that Hashem may not allow him to go with men of inferior standing such as them. 1
Seforno: He meant that he needed to prepare himself for prophecy.
Where were the elders of Midyan?
Rashi: Refer to 22:7:1:2.
Ramban #1 (on Pasuk 13): When Bil'am informed them that he needs permission from Hashem to curse Yisrael, they left because they knew that Hashem who took them out of Egypt and who performed with them miracles and wonders would never grant him the permission that he sought. 1
Ramban #2 (on Pasuk 13): It seems that elders of Midyan 2 came to see Balak in Mo'av to discuss with him the sudden appearance of Yisrael, and when they left, Balak sent his wise men together with them to invite Bil'am. When they arrived in Midyan, which was en route to Aram Naharayim, where Bil'am lived, the elders of Midyan remained and the elders of Mo'av continued alone on their way to Bil'am.