You said, “Her original sin was then forgiven in advance,”
Well .. if that was true, then there would have been no need for Jesus to come to forgive our original sin, if God could just wave his magic wand and make the sin go away.
That’s illogical .. and God is not illogical. He told Adam and Eve .. if you eat of that tree of good and evil, you will surely die. Well, Adam and Eve didn’t die - not physically .. but they died spiritually.
God made the law of spiritual death .. he couldn’t flip a switch and change his mind to rid Mary of her spiritual condition. Therefore, Mary could not have received a new spirit before she had Jesus. And .. the Book of Acts clearly tells that she did accept Jesus as her Savior on the day of Pentecost - and thereby received her renewed spirit like all the others who were there in the upper room.
I’m sorry you can’t see this truth.
He didn't rid her of it, He prevented it and in so doing did not violate any of His own laws.
Mary was preserved exempt from all stain of original sin at the first moment of her animation, and sanctifying grace was given to her before sin could have taken effect in her soul. "...was preserved exempt from all stain of original sin..." "The formal active essence of original sin was not removed from her soul, as it is removed from others by baptism; it was excluded, it never was in her soul.