(forgot his name so Zack Flan is his name now) Has red eyes, it shown in the previous chapter how the kid with blue eyes after exchanging got red. What if he wanted more seemless transitions so he had a family who now all have red eyes, no one would notice. But you dont have to be in someone life to manipulate them, easily this man couldve had a family who extended into being royalty after many years of him exchanging left and right, and then dipped. It couldve been a close friend, a distant relative. Someone that had a close enough relationship where after her husbands death and her mana calificación they could access her dorm and exchange. Cause they do have to be with the body for hours to pass all their mana. Maybe someone who lied and pretended they where looking after a "fainted" body. Like a doctor or a maid. Nanys usually stay with children for long periods of time and considering she was the youngest her nany couldve stayed with her longer.
Making up random people to explain random theories but yeah didnt have to be her parent since it is mentioned could be your own or anyone elses.
I feel like the same thing also happened to the empress but I so much hope that since it he past life no one really swapped with Ludwig it will be somehow possible to reverse the experiment and hope that there is going to be some more flashback about the empress and how she loved her children and even Ludwig.
But I wonder who swapped with the empress since by then both her parents and siblings are said to be dead