If in the general sense, Christ's death was necessary, this does not rule out that He can apply the merit of that death as He wills. As it is His death, and He is almighty, He can do what He wills. He can exempt His mother from the consequence of Adam’s sin. God is both perfectly just and perfectly merciful.
Then why did God not apply this merit to all the people thus avoiding the suffering on the cross? Why did St. Mary call Him "my Savior"? What are the Biblical proofs that support your argument?