It's all in the show. Londo is dead at the time of the party, about 3 or so years I think. The urn has one of those keepers in it. David opens the urn at 16 and is compelled to go to Centauri Prime, Sheridan and Delenn went after him and got caught and we saw them in prison. G'Kar it would seem was there to help Delenn and Sheridan, I guess, but didn't get caught. One can guess that was with the help of a changeling net if you follow only the show but you'd need to remember The Gathering for that. It is stated in the novels but the idea is hinted in the show. Londo did get his wish from Voice in the Wilderness, dying on his feet doing something noble, brave, and foolish.Ok, it's done. I have finished. First off let me say I really enjoyed the series on a whole. Great storytelling, interesting characters. I am looking forward to the movies, and the re-watch.
The finale... kind of disappointed. Felt lack luster, and kind of boring. Sorry. No Londo? No explanations about the urn he gave Sinclair for his kid? How did G'Kar end up killing Londo? Will these questions be answered later? I was expecting more.
But, maybe it will look better should you watch the series again. A lot of stories have a great deal of background but the hints and pieces are scattered throughout the show and not always part of a main plot so they can be missed. B5 is a show I find very rewarding to rewatch for that alone.