Avery probably killed her. I think the evidence you site points that way. I do find the finding of the keys after multiple searches odd, though.
But that kid’s coerced confession didn’t line up with the physical evidence. If they had cut her throats in that mess of a room, there would have been physical evidence.
Those detectives took advantage of a mentally deficient 16 year old and got him to say what they wanted to hear. And they did it without his guardian’s knowledge. the kid never had a chance, legally.
Possibly.