I'm ready to bet money that Eliot actually kidnapped and possibly brainwashed the mother, MC must be the daughter of some noble with unique charateriscs and would be recognized as her daughter instantly, very weird for him to send her in a place where it would be easier to discover her identity if she did one mistake but I'm 100% sure that Eliot lied.
The mother past was described as the cliche fall of a otome game villain bad end, but if she was really a commoner it couldn't be like that, she must be a noble to and is probably locked in the mansion, for that I think there is some brainwash at end.