I know how the contact was possible. I was wondering why Kosh stopped G'Kar's attack on Londo. I mean, it was the right thing to do, but Kosh really isn't out to stop what is, in comparison to the shadows, unimportant. He didn't need to stop G'Kar, but he seemed to do it to put G'kar back on track to go where he's been going, trying to tell people about the shadows by himself (which has been a goal of G'Kar's, but the narn/Centauri conflict has kind of derailed it). I'm wondering if Kosh is trying to turn G'Kar into an ally of the group fighting the shadows, or atleast remind him of whats important. G'Kar knows about the shadows, and was trying to warn people before he lost his position (and mentioned it to the earth rep investigating the footage of the shadows only a few episodes ago). I'm assuming thats also why he gave Garibaldi his book, which has an image of the shadows in it (although I don't know why he didn't just point garibaldi to the page). I'm sure its stuff that will be brought up soon, I was just kind of wondering out loud (well, not actually out loud, but you know what I mean).
All good questions....which I choose not to answer.
One small note though: that book G'Kar lent to Garibaldi was the Book of G'Quan and there's a lot more in it than just a few doodles of the Shadows. It's a holy book written by G'Quan himself; one of the Narn's major religious icons/prophets who lived about a thousand years ago and witnessed those events first hand. I think his reasons for lending it had more of a spiritual motivation. G'Kar's already told the council and everyone that would listen that the Shadows were once on Narn, it's not really news.