My understanding is that these questions are answered in the books; more specifically, the Psi Corps trilogy and the Centauri trilogy. But as I have not managed to get through the third Psi Corps book or the first Centauri book, I can't say for certain.

Most of these issues are resolved in the Babylon 5 books, which I have read. To elucidate, without spoiling the surprise:

The Keeper in the Centauri vase is dealt with in the Centauri Prime Trilogy (AKA Legions of Fire), written by Peter David (based on an outline by JMS). The final book, Out of the Darkness, deals with the death of Londo and the ascension of Vir (and his bride, whose identity will floor you!). It is in this series that Vir's allies help him deal with the Drakh.

Bester was scheduled to appear in an episode of Crusade. I've read the script, and it was dynamite! Bester's final fate is dealt with in the book, Babylon 5: Final Reckoning: the Fate of Bester, which according to my computer is still in print.

Lyta Alexander was supposed to be in a flashback secene during an episode of Crusade, showing her final death in an atomic blast during the Telepath War, but problems developed during negotiations, and another telepath was used. Her passing is mentioned by G'Kar in the TV movie, "To Live and Die in Starlight", which was going to be the pilot for a new series called, "Legends of the Rangers". Alas, it never materialized. Hollywood can be so stupid sometimes.

There are an awful lot of Babylon 5 books out there, and all of them have to pass muster with JMS (thank god!), because they are part of the official timeline. So, read, and enjoy!