Suas presunções estão corretas. Dumbledore fez um show de assassinar Narcisa.
Em algum momento durante a Guerra Mágica, os Comensais da Morte começaram a perseguir as famílias. O irmão de Dumbledore foi mantido como refém e torturado até a morte sob a maldição Cruciatus. Em resposta, Dumbledore "matou" Narcisa.
"Do you also wish to see my brother as he died under the Cruciatus?" said Albus Dumbledore. "Voldemort sent me that memory as well!"
"And that - " Harry was having trouble producing a voice, for the growing sickness in his chest. "That was when -" The words seemed to burn in his throat, as the awful knowledge dawned on him, the horrible understanding. "That was when you burned Narcissa Malfoy alive in her own bedroom."
Albus Dumbledore's gaze was cold as he answered. "To that question only a fool would say yea or nay. What matters is that the Death Eaters believe I killed her, and that belief kept safe the families of all who served the Order of the Phoenix - until this day. Now do you understand what you have done? What you have done to your friends, Harry Potter, and to any that stand with you?" The old wizard seemed to grow still taller and more terrible, as his voice rose louder. "You have made them all targets, and targets they will remain! Until you prove, the only way it can be proven, that you are no longer willing to pay such prices!"
"And is it true?" Harry said. There was a buzzing sensation filling him, his body growing more distant. "What Draco said, that Narcissa Malfoy never got her hands dirty, that she was only Lucius's wife? She was an enabler, I get that, but I can't back that deserving being burned alive."
"Nothing less would have convinced them that I was done with hesitation." The old wizard's voice brooked no question and no refusal. "Always I was too reluctant to do as I must, always it was others who paid the cost of my mercy. So Alastor told me from the beginning, but I did not listen to him. You, I expect, shall prove better at such decisions than I." (Chapter 82)
Por causa da resposta de Dumbledore, os Comensais da Morte perceberam que não tinham nada a ganhar e muito a perder continuando a perseguir as famílias de outras pessoas. Então eles pararam. É por isso que Dumbledore fez isso.
Não há nenhuma indicação qualquer pessoa que Dumbledore saiba que Narcisa está viva, especialmente Lucius. Lucius era uma das pessoas que Dumbledore tinha que parar de fazer isso novamente. Isso derrotaria completamente o propósito de Dumbledore se Lucius soubesse.
Que Narcisa foi salva foi prefigurada aqui, e quando Harry conheceu Dumbledore pela primeira vez.
"I didn't know about Fawkes," Harry's voice said rapidly, "so he told me that Fawkes was a phoenix, while he was pointing to a chicken on Fawkes's stand so I'd think that was Fawkes, and then he set the chicken on fire - and also he gave me this big rock and told me it had belonged to my father and I ought to carry it everywhere -" (Chapter 70)
Dumbledore descreve isso em sua carta para Harry como:
"Here's the last thing," Harry Potter said. "I found it in a folded parchment whose outside said that it was the last weapon to be used against House Malfoy, telling me not to read any further until the whole war hung in the balance. I didn't want to tell it to you before because I thought it might prejudice your decision unfairly. If you were a good person who never killed or lied, but you had to do one or the other, which would be worse?" (Chapter 120)
... o que confirma para nós Lucius definitivamente não sabia. Se Lucius já estava sendo chantageado, não seria a "última arma" a ser usada contra House Malfoy: é um coelho que você tira de um chapéu quando não sobra nada, o que vem com algum risco, porque Isso mostra que você nunca esteve realmente disposto a retaliar em espécie afinal.