Could He then have also applied them to everyone else and spared his Son the Cross?
He has the capability as God, but according to His Will, only His Mother was spared, so that she would be a fitting tabernacle for His Son, Our Lord Jesus Christ. If the Ark of Covenant, which containted the written Word of God had to be built according to God's specifications, how much more important that the tabernacle of the Word Himself be built pure!
Those merits were purchased at the Cross and because of it. However, since God exists outside time, he is not bound to apply Christ's merits only temporally after the Crucifixion. He can apply them to Mary beforehand.
Far from asserting that Mary somehow did not need a Savior, the I.C. asserts that Mary is the perfect example of redeemed humanity. We are not celebrating Mary's holiness apart from Christ's work, we are instead celebrating Christ's work in Mary to accomplish that holiness.