"Without Mary's "yes," we would not have Jesus. Do you have trouble understanding that?"
If we did not have "Mary" we would have had a "Hannah" or a "Sarah" or someone else. Mary could not prevent the salvation of mankind.
There is no one among all the inhabitants of the earth who can "stay His hand". No human can prevent God from fulfilling His plan.
"If we did not have 'Mary, we would have had a 'Hannah' or a 'Sarah' or someone else."
But then we would not have had Jesus; we would have had some other man. He took his human nature from Mary.