Could Mary have stopped the crucifixion?

Mary was human, not divine, so she did not possess the power to miraculously stop the crucifixion from happening. Like the other witnesses of Jesus’ death, she could only stand by, watch, and (surely) pray for her son as he suffered.

If your question is asking whether she could have spoken to the authorities and halted their plans, it’s unlikely that such a thing would have happened. The only one who truly had the power to either order Jesus’ execution or to set him free was Pontius Pilate, the Roman governor of Judea. The Gospels indicate that he was influenced in his decision by religious leaders and by the crowds, but given the fact that less than a day passed between the arrest of Jesus and his crucifixion, it is unlikely that Mary was ever given the opportunity to plead her son’s case.