Question: At the end of the movie, the Voice is credited as Himself. Who was really responsible for doing the voice that sent Ray on his journey?
Answer: It was actually his own voice (Kevin Costner) that sent him on the journey. At the end of the movie there's even the part where Ray is recalling the voices he heard and turned to Shoeless Joe Jackson and says, "It was you". At which point Joe Jackson turns around and says, "No Ray. It was you".
Question: How come none of the ball players can go past the first base foul line?
Answer: If they go past that line they will no longer be able to play again. The field is their only place to exist in the afterlife (their heaven if you will). That line is the end of the field and they will not be able to return if they cross it.