Well, from that point the Centauri are aware of his 'allies' and it is likely that when he pulls back from using the Shadows another will come forward. So it's pretty likely that Centauri Prime is doomed to have Shadows on it, but Londo's exact part in that is still to be decided.

I've always felt that the Coming of Shadows decision was understandable, albeit mistaken in it's assumptions. Of course, Londo himself gets treated to a (bitter) taste of what might have been when G'Kar buys him a drink before word of the attack gets out. Londo's damnation comes when he continues to use the Shadows despite past mistakes and with full knowledge of what he's unleashing on the Narn as a direct result of his actions. Up to then he's still redeemable, except (probably) in Vorlon eyes.