I meant the time in The Coming of Shadows - that, to me, is Londo's crossing the line moment. The Narn homeworld obviously is another big step, but in Coming of Shadows he starts a war, and while, yes, we know the Narns are itching for one as well, this doesn't excuse Londo taking that step. The text itself is pretty explicit about this, too, given the Vir-Londo dialogue ("you don't know what you're doing!" "Yes, I do. Yes, I do" - as opposed to Chrysalis, when he didn't), and the fact that this is when Londo sees the Shadow ships over Centauri Prime for the first time in his vision. (We know he has seen his death before - the mutual strangling - but he had not seen the much earlier set vision of the Shadows over Centauri Prime, which I think is due to the fact that only after he made the request to Morden - and helped bringing Cartagia on the throne - did the Shadows on Centauri Prime become an inevitability.)

"they are a dead people" (re: the Centauri and the Narn)

Finally, some one who gets this the same way I do. Everyone always uses that quote specifically for the Centauri. I always thought the point of that quote was that from the Vorlon perspective both races were doomed, with little difference between them (thus hinting at the war to come being based on ideology, not species).

Absolutely. I think Kosh the individual later comes to care about G'Kar enough to intervene in Dust To Dust, but the Vorlons in general have written off both Centauri and Narn alike.