Marley was stuck in an eternal loop of suffering because he couldn't affect the world: "The misery with them all way, clearly, that they sought to interfere, for good, in human matters, and had lost the power for ever.”
But Jacob himself said that the chance that Scrooge was given was “a chance and hope of my procuring.” When Ebenezer changed, his actions affected the world, for good. And since Jacob caused the change of heart, indirectly, he affected the world in a positive way as well. His love for Ebenezer proved stronger than the chains that weighed him down.
In other words...by doing one selfless act, in attempting to save his friend from his own fate--an action borne of pure Love, which would, he believed, not profit him but only others--Jacob, unwittingly, redeemed himself also.
My headcanon continues thus:
After Scrooge died, Jacob and Scrooge continued the good work from their own realm. They did this in prompting the living, through dreams, circumstances, or simply feelings that sort of popped into people's minds (assuming they were open to the ideas). Spirits can have a lot of influence in the mortal world...if we let them.
But there’s more. Before he passed on, Scrooge had told Tiny Tim, when he was just Tim, being a young adult, the story of what happened to him that night. And Tim told one or two people, and THEY told one or two people. And the story grew and spread its light and warmth, like a flame in the darkness, and people began to forget it ever really happened...until one day, a man named Charles Dickens heard it and thought it would make an excellent book.
As the tale grew more famous, Jacob and Ebenezer became well-known in the living world, and the mention of either of their names now brings the thought of Christmas instantly to mind...and so, they have actually become Spirits of Christmas in their own right—the same as the ones who visited Scrooge that long-ago night. Turns out that some Christmas Spirits always were and always will be, but others are made...if someone mortal passes that far into the collective consciousness of mankind in association with Christmas, they become a Spirit. (This also happened to a living man...who is now immortal, and goes by the name "Santa Claus.") The irony of this is not lost on either of the two Spirits, and the source of much laughter between them. But neither of them complains...it makes their work easier, after all.
And so, it fits that Dickens opened with the phrase “Marley was dead, to begin with.” Because Marley’s death was just the beginning, for both men.
If anyone else is interested in Jacob's possible redemption, I'd recommend:
-"Jacob Marley's Christmas Carol," a play by Tom Mula
-"Jacob T. Marley", by R. William Bennett
and last but not least,
-"A Conspiracy of Spirits: The Love Story of Jacob Marley and Ebenezer Scrooge," found in collection "The Drosselmeier Chronicles: The Solstice Tales" by dA's very own A sample of this one can be read here: wolfenm.deviantart.com/art/A-C… (Please note that this one has strong themes of gay romance, so if you're not interested in that, don't bother the author, k?)