Isis was more popular than Athena. Besides the warrior image of Athena/Minerva did not mesh well with the image of Mary, and the relation of Isis to Horus better matched Mary to Jesus. The image of Madoona and child Jesus seems to be copied from the images of Isis with a young Horus. (Not surprising - painters who made a living painting the Isis and Horus, just switched over to painting Mary and Jesus, and painted what they knew. Everybody was happy. It wasn't until many centuries later you got iconoclast in the East, and Puritans in the West that objected.)
That Mary was a virgin when Jesus was born is part the canonical gospels and so is dogma. That ir was believed Mary remained a virgina all her like, like Athena, is not necessaeily the case for early Christians. The Gosepls don't say anything about Jesus' brothers and sisters being only half brothers and sisters. While Catholics and Orthodox believe Mary was a virgin all her like, Protestants don't, and are ok with Jesus having full flooded siblings.
I think you forgot we were discussing the Eleusinian mysteries and Persephone's virgin birth of Aion, not Isis. The reason for copying it would be to raise Persephone's status towards her Virgin neighbour goddess Athena and maybe Artemis too. Though I am speculating too much for such shallow evidence and it is even a childish reasoning I'm making.
BTW I agree that Mary and Isis embody the similar "mother of God, mother of all" attributes and Athena embodies divine wisdom and so they are about different things.