im so confused, apparently anyone with the original keys could have signed that blockchain message to Hal.

doesnt that mean the keys "have" been used? I thought the original keys had never been used? Or is it just no money has moved?

What about Len Sassaman? isn’t he considered a strong candidate also?

He was critical of blockchain, because he considered a permanent register of transactions to be the antithesis of privacy. How much store you want to set by that is up to you, but my instinct says that he was sincere.