I see four possibilities:
* An absurdly elaborate conspiracy. (This is always a possibility.)
* Ghosts can do cryptography.
* ECDSA is cracked.
* Someone else has a copy Finney's key.
Occam's razor: spectral signatures!
I'll be more sober about it. A cryptographic hash is not proof that data was signed by a particular individual, unless you know for certain that individual is the only person who has the key. That is a hard standard to meet with a dead man when the key has demonstrably been used after his death. That transaction in 2017 means that someone besides Finney simply must know Finney's key. Anything signed by this key cannot be trusted to have come from Finney.
someone who isnt paul le roux left their signature saying it was paul le roux, in case it ends up being read one day
(1) who has Hal’s key
(2) where did Martin “find” this message to be able to decrypt it
There’s obviously some sort of agenda here involving multiple people, it’s just not clear at the moment what this particular agenda is.