You are missing the point Lace. Isn't the choice of Madeleine, with her talkative nature, her age and her distinctive eye, rather a strange one for adoption, when her sister would have had none of those drawbacks and was there for the taking ?
Taking Amelie would not have necessitated drugging the children as she would not have been able to raise the alarm in the same way as Madeleine and her identity would have been easier to conceal as she was carried away.
Somebody wanted Madeleine. Why, we do not know. Certainly her front door was VERY vulnerable and a watcher could oversee who was coming and going.
Blonde does seem to be important. All 4 girls who have been abducted were fair haired. Rene Hasse was also blonde, but the older children, all boys, were more latin in looks.
Personally, I think that Madeleines smiley face and intellect shines thru as does her sense of fun. Maybe someone wanted these attributes. We dont know and can only guess tbh.