I hadn’t realized that there was such an easy way to bring back lost memories. More reason to put that emerald ring into the Mirror, ASAP.
I took it to mean that Dumbledore had used a reversible memory charm on Narcissa, knowing they might want her back once Harry had defeated Voldemort, or, more practically, they could use her as a “last weapon to be used against House Malfoy” by holding her hostage.
I think Obliviate is still permanent for all practical purposes, so the same couldn’t be done for Voldemort.
I took it to mean that Dumbledore had used a reversible memory charm on Narcissa, knowing they might want her back once Harry had defeated Voldemort, or, more practically, they could use her as a “last weapon to be used against House Malfoy” by holding her hostage.
I think Obliviate is still permanent for all practical purposes, so the same couldn’t be done for Voldemort.
[edited iteratively]