Okay I will ask again.
If God could mysteriously (since her parents were not sinless) cause Mary to be born without original sin then why couldn’t God just have allowed Jesus to be mysteriously born without original sin even if Mary was born into original sin?
That is a great question. I think (I am not answering for the Catholic Church) it has to do with the hierarchical relationship between Mary and all other women including Eve. Mary had to be greater than all other women, including Eve who was conceived immaculately from Adam's rib. For most of human history the natural relationships among humans was to have exalted royalty. To introduce Mary and Jesus to the world, at that point in human history, in any other state would have made an already difficult task even harder.