Does the canon explicitly address which of the following 3 acts cause a piece of Voldemort's soul to split off and embed into Harry?
- Murdering Lily
- Trying to murder Harry (e.g. would the soul have split even if he didn't get hit by rebound Avada Kedavra?)
- Getting hit by rebound Avada Kedavra - if he didn't, the soul piece would NOT have split off.
Canon means books, JKR interviews or Pottermore.
I'm very specifically interested in information OTHER than Dumbledore's guess stated in chapter 24 ("The Prince's tale") of Deathly Hallows: ...
“Tell him that on the night Lord Voldemort tried to kill him, when Lily cast her own life between them as a shield, the Killing Curse rebounded upon Lord Voldemort, and a fragment of Voldemort’s soul was blasted apart from the whole, and latched itself onto the only living soul left in that collapsed building. Part of Lord Voldemort lives inside Harry, and it is that which gives him the power of speech with snakes, and a connection with Lord Voldemort’s mind that he has never understood. And while that fragment of soul, unmissed by Voldemort, remains attached to and protected by Harry, Lord Voldemort cannot die.“
... and Chapter 25, "King's Cross":
“You were the seventh Horcrux, Harry, the Horcrux he never meant to make. He had rendered his soul so unstable that it broke apart when he committed those acts of unspeakable evil, the murder of your parents, the attempted killing of a child. But what escaped from that room was even less than he knew. He left more than his body behind. He left part of himself latched to you, the would-be victim who had survived.”
The reason is that this quote doesn't seem to very clearly point to causality, and also it was just Dumbledore's guess. I'm hoping one of the JKR sourced information sources would clarify this.