I propose that Dumbledore helped to fake Narcissa's death so she could join the light-side.

I'm not sure if Eliezer has read the sixth book, but Dumbledore makes this very offer to Draco:

"I can help you, Draco."

"No, you can't," said Malfoy, his wand shaking very badly indeed. "Nobody can. He told me to do it or he'd kill me. I've got no choice."

"He cannot kill you if you are already dead. Come over to the right side, Draco, and we can hide you more completely than you can possibly imagine." (HBP, 591)

Dumbledore proceeds to offer protection to all of Malfoy's family.

Daniel_Starr mentions below that Narcissa might be unwilling to leave her son with Lucius if she's defecting. So there would have to be some pressing need to disappear in order for this theory to hold water; her life would have to be in danger, or something similar.