I have the Monkey Key and the Serpent Key, but I haven't found the Keys for Kerah (Kol Key), the Key to the room before elevator maze and the Key for the elevator! Where do I find them? Or is there a way to complete the task without the Keys?

You need the SKULL key which is held by the Serpent Protector. He is somewhere around the elevator puzzle. The key opens the locked room in the maze after you kill Xydussa and the exit door. Can't remember exactly where he is. Maybe someone else does.

Kol should have given you his key when you talked to him.