I’m not saying I think Narcissa is alive, but...
There’s no obvious reason not to return her after he thought Voldemort was gone
Except that Narcissa could then testify in front of the Wizengamot that Dumbledore kidnapped her.
or at least to let Lucius know what happened in case she’s alive and didn’t want to return
Dumbledore believes Voldemort will return. This would limit his ability to threaten Death Eaters in the next war.
I downvoted the previous post because it was a needlessly complicated, poorly justified plan. Crucially, there was little indication of why Voldemort would want to pretend to lose, when he was already winning the war. By contrast, your more recent post is a good analysis of the new insight into Voldemort’s history and motivations provided by the latest chapter.