To say no to the Drakh might have been the redeeming answer. The price would have been terrible, but after that Centauri could be rebuild free of the Drakh influence. (As G'Kar realized: "Some must be sacrificed for all to be saved") Unfortunately Londo's decision did not really save that many people since the bombs get detonated 17 years later.
I believe this was his greatest fear. Throughout the series Londo expressed his desire to see Centauri return the greatness it once had. That desire, along with his own ambition, led to the Shadow influence on his people in the first place. If he had flatly said no, the Drakh would have detonated the bombs killing millions if not billions. And that is exactly what he should have done. Sheridan and the alliance would have come to their aid and finished off the Drakh.