But faking her death (and even the type of death) doesn’t really match the rest of the story. There’s no obvious reason not to return her after he thought Voldemort was gone, or at least to let Lucius know what happened in case she’s alive and didn’t want to return—which is unlikely, we had no indication that she was really unhappy or didn’t wish to be a part of her son’s life—or if she died in some much-less-objectionable way (he could give Lucius the memory).

But he never thought Voldemort was permanently gone in the first place. The Hogwarts inner circle knew Voldemort was still around as of the beginning of the story. And in any case, he's still maneuvering against Lucius, so he has an incentive to uphold the notion that he will not cooperate with blackmailers and that he will resort to equal retaliation, even while his greater enemy is absent.