Next easiest: Harry goes back 5 hours, sends a note to original Harry explaining exactly what he has to do to save Hermione; original Harry does the heroics, including setting up a fake death and writing the directions he got, while future Harry executes the actions in the chapter, failing to prevent the death. There’s a maneuver somewhere in there where future Harry then snaps his fingers, goes back one hour, and somehow initiates the course of events described (probably by blackmailing time).
The theories have now gotten more unlikely than that Harry got the timey-wimey ball and is prohibited by an unstated rule from preventing a plot event.
Next easiest: Harry goes back 5 hours, sends a note to original Harry explaining exactly what he has to do to save Hermione; original Harry does the heroics, including setting up a fake death and writing the directions he got, while future Harry executes the actions in the chapter, failing to prevent the death. There’s a maneuver somewhere in there where future Harry then snaps his fingers, goes back one hour, and somehow initiates the course of events described (probably by blackmailing time).
The theories have now gotten more unlikely than that Harry got the timey-wimey ball and is prohibited by an unstated rule from preventing a plot event.