WARNING: One giant spoiler.
Summary and comparison of various interpretations:
Did Londo ultimately suffer the fate foreseen by the Lady and the Technomage?
It is plausible that he failed entirely to avoid this. At the time of his death (in 'War Without End'), we see the capital city of Centauri Prime literally on fire outside the palace window. And by that time he arguably had, at least, millions of victims over his lifetime. Billions of victims is plausible, as much of his time as Emperor is off camera. He killed many Centauri, Narns, humans, and other aliens, by his own hands, on his orders, or as a direct result of his schemes.
But still, had he not asked G'Kar to kill him, and the Drakh keeper along with him, a still worse fate may have awaited him.
What was "the eye that does not see"?
There are several noteworthy interpretations.
- As noted by @TML, Straczynski the show's creator hints that the "eye" is actually a pun, a reference to "I, myself". The idea is that Mollari himself 'does not see' his own situation clearly until it is too late. As shared by @Radhil, Mollari himself comes to believe this is the correct interpretation.
While it does come from the show's creator, this idea also reeks. It is hard to swallow that, in universe, a Centauri prophecy should be interpreted according to English (an alien language to them) sound-alike phonics. Even though a semi-canon novel places the idea in Mollari's own words.
- @Izkata A precious Centauri artifact from season 1 called "The Eye".
This is not plausible simply because the episode with this artifact happened before his consultation with Lady Morella, and never again becomes relevant to the plot. His chance to "save the eye that does not see" is clearly a future opportunity for him, not a past one.
This seems to be the best and most obvious idea but has its drawbacks. Had Londo intervened to save G'Kar's eye, or have him released, it would have changed the course of history within the Centauri court in unknowable ways, possibly arresting Londo's moral descent. Also, as shown by @Radhil, the show's creator has dropped hints at this interpretation.
But it is not clear that this qualifies as a solution. Physically, G'Kar could see just fine before his eye was taken. His process of philosophical enlightenment was already well underway by season 3, so it is not clear that G'Kar 'could not see' for lack of moral or spiritual clarity at that point in time. However, his ordeal did precipitate a new stage of his enlightenment.
It is also not clear whether saving his eye would have been better for G'Kar in the long run, or whether that even matters for purposes of Londo escaping his own darkness. G'Kar became open to the idea of genuine friendship between the Centauri and Narn races only after his ordeal, and therefore would likely not have become good friends with Londo.
Who was "the one who is already dead"?
- One of the popular ideas here is Sheridan, since he dies a real death at Z'hadum at the end of season 3. For the rest of the series, Sheridan is "already dead". Londo arranges for Sheridan and Delenn to be freed, and in exchange, they will help to free the Centauri from the Drakh menace. Mollari even describes this arrangement as a "redemption" in their brief meeting.
There are two problems with this. The first is that Londo has the choice to kill Sheridan only at the very end of things, as Centauri Prime is already burning, as Londo's victims already number beyond count. It is too late to avoid this situation, which is already a nightmarish and unenviable end to Londo's life. At this point, it seems the "billions of victims" fate foretold by the Technomage has already happened.
The second problem is that the Sheridan who stands before Londo in War Without End is not, in fact, "already dead". That Sheridan has not yet been to Z'hadum. Delenn in fact tries to talk him out of it. While the Sheridan of that time is still around, it is not that Sheridan who faces Emperor Mollari in the throne room, but the 'un-stuck in time' Sheridan from years earlier.
- This page states the opinion that "the one who is already dead" is Morden. Londo did in fact kill him (by order) in season 4. Morden was under the mind control of the Shadows, a form of death. Morden was officially declared dead by Earth years prior. Morden fits.
The major problem with this idea is that it's really hard to see having Morden killed as the wrong decision, especially given the circumstances under which it was done. Nor is it clear how not doing this would have allowed Londo to escape his fate.
- I think "the one who is already dead" is more likely to have been Lord Refa, the craven courtier whom Londo blamed for the death of his true love Adira. Refa has been half-poisoned by Londo prior to the Lady's prophecy. Later, in seeking his revenge for Adira's death, Londo commits himself to the Shadows and corrupts the Centauri Republic.
Everyone around me dies, Mr. Morden, except the ones that most deserve
it. That is about to change. You said that you would go away for as
long as I wanted. I no longer want that. All I want now is revenge.
They took from me the one thing that I have, have ever truly loved.
And you will help me, Mr. Morden, to strike them down! Give me this, and
the safety of my people, and let the rest of the galaxy burn. I don't
care any more.
Will you help me, Mr. Morden?
Resisting the temptation to assassinate Refa and humiliate his faction, with the help of Morden's 'associates', could plausibly have allowed Londo to escape his fate. Before he restarted his relationship with Morden, he had successfully broken the influence of the Shadows over the Republic. This indulgence in revenge was a critical failure and a fateful one.
The major weakness in this theory is that Refa may not qualify as "already dead". It may not be enough that he was half-poisoned; or it is possible that the poison was simply a bluff by Londo. As Refa was killed another way, and Londo never confesses to a bluff, there is no way to know for sure. But for what it's worth, it doesn't feel like a bluff, as there was no private talk with Vir where Londo brags to him about bluffing Refa. Even Morden was told about the poison and apparently believed it (Interludes and Examinations).
Also, while Refa is certainly an amoral Machiavellian of the highest order, that's no reason to believe a Centauri prophetess would consider his cravenness to be even a metaphorical form of 'death'.
- "The one who is already dead" could even be Emperor Cartagia, who was certainly insane (his mind was "already dead"), and was assassinated by a conspiracy led by Londo. His assassination led directly to Mollari becoming Prime Minister, and later Emperor. This is another situation where it's difficult not to sympathize with Londo's decision though. And had Cartagia not been assassinated, would that have been better or worse for the Republic?
Did Londo "surrender himself to his greatest fear"?
It is not clear what his greatest fear was. In season 1 Londo had a prophetic, and accurate, vision of himself being strangled by G'Kar, which appeared to unsettle him greatly. But whether his greatest fear was death at the hands of a Narn is less clear.
He either never surrendered to his greatest fear, and suffered his foretold fate; or he avoided his fate without having to do this; or he narrowly escaped his fate by inviting G'kar to kill him. In the last case, that would make his greatest fear to be something along those lines (giving up control, death at the hands of a Narn, etc).
What were the two missed chances?
- @Tyson_of_the_Northwest offered Morden's offer to Londo to "just pick a target". This is clearly a critical over-indulgence in the hunger for power by Londo, that leads him down the path to corruption and power.
- Another key failure is when the Centauri ministers demand that Londo rely on Morden's "associates" to start open war between Centauri and Narn, and he does so, burning his relationship with Babylon 5 management in the process. This act cements Londo as an asset of the Shadows, whereas before the Shadows through Morden had been willing to play as Londo's asset.