I'm inclined to agree on the Sorting Hat. The sword is a red herring. Or, rather, a decoy. We only need to find out whether there was any opportunity for Tom to have gotten hold of it at the time that a death took place.

Dumbledore's identification of Nagini is a mistake. Voldemort was barely back in the physical world at that point and i think that he was too weak to create a Horcrux until after he re-incarnated himself at the end of GoF.

Besides; merely murdering someone does not create a Horcrux. It messes with your soul, but in itself it doesn't peel off a piece so you can secure it into an object. A simple AK won't do it. And we've mostly just assumed that Harry survived the AK. But if Voldemort did intend to create a Horcrux with the death of his "appointed enemy", what he threw at Harry was not a simple AK, and he had already brought an object with him to hold the soul fragment.

However, when the spell *found* a victim, it wasn't the one that Voldemort expected. His soul got a piece sliced off it when it left his living body and he was in no condition to direct where it went. I doubt that the Horcrux is Harry himself, but It has a good chance of being the curse scar.

Most unfortunately, the false lead on Nagini suggests that Harry will be face to face with the enemy before he discovers this.