Because she chose not to. She had other things to do with her life.
Certainly not. For one thing, Yoda was dead by then, and Force Ghost or not, I doubt Luke had any illusions that he was going to be training Leia from beyond the grave.
No, Luke simply meant that the strength and special capabilities Leia saw in him were also in her, and that he was confident that the Rebels wouldn't need his help in order to complete their mission, because they'd still have Leia with them. He's not treating her as a backup in that scene, he's saying that he himself is the redundant one in that scenario - they can complete the Shield Generator mission without him, so it's okay that he has to leave in order to confront and redeem Vader.