It's certain that she died during the Telepath war and it *seems* conclusive that this was the incident since in the Crusade Writer's Bible, JMS wrote that she and Lennier die during the TW and the original draft of the script that featured her has a back view of a Minbari fighting with a pike and she calls Lennier's name just before the explosion.
So what the hell happened, anyway? Did Billy Mumy punch JMS' mom in the mouth, or bluster in while JMS was having an unfortunate bowel movement, which he then recorded and played over the PA system? Did he GET the part because he had pictures of JMS naked with a goat, and then get booted once JMS's detectives managed to burn the pictures and negatives?
I ask because, seriously, SOMETHING must have happened to make Mumy fall from favor. He's all over the place in Seasons 1-4, and though he's not the star, he is definitely a shining light, and he just brought so much to every scene. No offence to Stephen Furst, who did a great job, but Mumy was just chocolate-coated chocolate awesome with chocolate filling. And it was obvious that Lannier was being built up for some greater destiny. He had a future in front of him. It was probably something he wouldn't survive, but it was important.
Annnnnnnnnnnd then he's barely in Season 5, and he betrays his own character in a clumsy change of heart/change back scene. And then he's re-cast as an extra and dies with barely a namecheck.
Seriously: What the frack?
I mean, something MUST have happened.
Also, I'm not buying for a minute that the original plan called for him to die in the Telepath war. Dying rescuing Delen and Sheridan, that I could buy, but being bumped off like that in Crusade?
Nope, nope, nope, not buying it. Something musta' happened. JMS gets pissed, he kills your character.