Since Lady Morella spoke of Londo's "last chance of redemption", I think this is more about Londo himself, or rather his soul, than about the fate Centauri Prime. In the spirit of "Comes the inquisitor", it's about doing the right thing, making the moral choice, regardless of whether it serves to save millions, or just one person, regardless whether one becomes a hero, or the choice remains unacknowledged.
Therefore I think that the part concerning "saving the eye that does not see" indeed refers to G'Kar's eye, and like the scene in "The Very Long Night of Londo Mollari" when G'Kar was whipped, it doesn't matter if it had succeeded in stopping Cartagia - Londo was there, and he had an obligation to speak out. Same as he had an obligation to save Sheridan, whom he knew to be innocent. As for "his greatest fear", I think it's surrendering to the Keeper. All Londo ever wanted was power, and the freedom to choose, and friends. And surrendering to the Keeper meant he lost all of that, and became almost a puppet. He lost all he ever wanted, knowing he would never get it back.
Along the same line of thought, I think the first two chances (the ones he wasted) weren't about stopping the "big events" either, but about making the moral choice in personal situations. My guess would be the incident with Urza Jador, and the treatment of G'Kar in "The Long Twilight Struggle", when he humiliated him in the council chamber, forcing him to resign as ambassador and trying to send him back to Narn.