My interpretation, Murph solved the gravity equation all by herself. Plan A was a success. She never really got over her father leaving her.

Probably he couldn't take it and just left. Maybe, they flew into the black hole and died. But she's a genius and needed closure. On her deathbed she had a fantasy or hallucinations about what could have happened. In any case, her super logical mind came up with a story that let her come to grips with the loss of her father. It's a bit grim, but gives enough room (for me) for lots of silliness. that's not what happened. they all died. But maybe, that could have happened. And that's enough for her to find some peace.