If I accept the DaddyPan theory I think this is how it plays out:
–YoungRumple meets a man who gives him Pandora’s box and says it will help his father become less cruel and ruthless and will reunite Father and Son
–Rumple uses the box on his Papa except the evil inside the box infects MalcolmPan and his father runs away, Rumple thinks he has been abandoned
–DaddyPan comes back to Rumple who is now living with Spinsters. But this time Malcom has deaged into Peter Pan
–Peter Pan realizes he will grow up and flees to NL, leaving Rumple behind once more. Rumple now feels abandoned by both Malcom and Pan, not realizing that they are both
–MalcomPan becomes darker and more devious and more determined to live forever as a youth as the evil from Pandora’s box continues to work on him
–The only thing left in Pandora’s box is HOPE which will “save” Pan but only traps him inside the box
–They go back to SB where Pan is somehow released (Blue Fairy?)
–Rumple realizes the only way to defeat PP is by stabbing him with the Dagger, giving up his own immortatlity in the process.
*new headcanon*