Well, my argument is that the internal logic of the story supports me.
Lady Morella says that Londo has three chances. Save the eye, don't kill the man, surrender to his greatest fear, we all know them. Also, remember that if he blows the first two the third chance is his only chance for redemption. We're pretty clear that G'Kar's eye was the first. So that's settled.
But if Sheridan were the man who is already dead... and not killing him redeemed him... then by simply not killing Sheridan Londo is redeemed, the third chance for redemption is unnecessary, Londo doesn't have to surrender to his greatest fear and be destroyed by it, and we encounter the very first time in the entire B5 universe where something doesn't happen in threes. In fact, it could be argued that since Londo doesn't need to be destroyed, he should survive and die peacefully in his old age, surrounded by friends and relations.
Except, of course, he doesn't. (Instead he dies surrounded by one friend. Or at least his neck does.) His death is necessary for Sheridan's escape -- and Delenn's, too. Let's not forget her. So either this interpretation of the prophecy is a little off, or Londo is caught in some cosmically tragic Catch-22. "Yeah, you've already been redeemed, so you don't have to surrender to your greatest fear and be destroyed by it. But you still have to die."
As I see it, the only way you can say that Sheridan is the man who is already dead is if you also say that Londo got it right the second time and was redeemed, and the third chance at redemption -- now superfluous -- is something we never actually saw. Which doesn't sit well with me either.
Now a good debater will present his counterargument, so here goes:
Morden is the man who is already dead. What Londo should have done, in order to both save his world and to avoid succumbing to his own desire for revenge, would be to nuke Selini and then send Morden off to the other Shadows with Londo's message: hands off Centauri Prime. Instead, Londo kills the Shadows to get them off his world, but he kills Morden largely out of personal vengeance for Adira, and this, I would argue, is where he blows his chance for redemption.
Londo surrenders to his greatest fear when the Drakh place the keeper on him. Consider his reasons for allowing this: he is doing so to save his world and his people, and is sacrificing his freedom -- not only of deed but also of thought -- for that purpose. Selfishness and ambition dragged him into the alliance with the Shadows (witness "The Coming of Shadows"), and now selflessness and sacrifice absolve him of that guilt. Also, consider: Londo appears to be a happy drunk in the early seasons, but this covers his own dread of his irrelevancy, his powerlessness. Through the help of the Shadows he shook off that irrelevancy. By surrendering to the keeper, he returns to that powerlessness and irrelevancy -- but he can no longer even control his own words or actions.
Moreover, being killed by G'Kar is no longer Londo's greatest fear at the time of death. He tells G'Kar to do it; he has been expecting it for a long time; by doing so he saves his people. It is something to embrace, not reject, and he embraces it.
So there's my argument: Morden is the man who is already dead, the keeper (loss of control) is Londo's greatest fear, he is redeemed by that act, and then marks time for seventeen years. When the chance to free his people comes at last, he jumps for it.
I really think JMS either missed a trick, or simply spaced out when he said Sheridan was the man who was already dead. Either way, that's how I read the prophecy.