1) mostly so that he can train Leia.  He plans to come back, but helping the Rebellion and training someone to help the against Darth Vader is more important.
2) I was planning on him giving her the same "Anakin Skywalker was you real father" speech and give her the light saber.
3) see #2

Edit: That said, if someone wants to play as Luke, I could have him say that he wanted to pick up "a new recruit" first and come to the farm to recruit you.