I found a good argument for the Immaculate Conception here: How Commonly Held Christian Beliefs Explain and Defend the Catholic Doctrine of the Immaculate Conception
Scenario #1 is that Christ – our Redeemer and Savior – inherited from His mother a pristine human nature which was not fallen. Her nature was "saved" from fallen-ness prior to Christ’s conception. In this scenario it is Mary who is the object of prior salvation. This is the Catholic view.
OR
Scenario #2 is that Mary was fallen at the time of Christ’s conception, but that Christ was prevented from (i.e., "saved" from) inheriting her fallen-ness through the intervention of the Holy Spirit. In this scenario it is Christ who is the object of the immediate saving work of the Spirit. This is the Protestant view even if few Protestants ever think of it in these terms.
In both scenarios we have the necessity of a "saving" intervention by the Holy Spirit.
But, in scenario #2 we can see a great difficulty arising. Why? Because for Jesus to have been "saved" from inheriting Original Sin that means that Our Savior would have needed a savior AND Our Redeemer would have needed a redeemer and THAT is something that Scripture never points to and that, in reality, is an utter impossibility. For Christ Himself is "THE SAVIOR" and He is "THE REDEEMER".
"Scenario #1 is that Christ our Redeemer and Savior inherited from His mother a pristine human nature which was not fallen. Her nature was "saved" from fallen-ness prior to Christs conception. In this scenario it is Mary who is the object of prior salvation. This is the Catholic view."
No. Jesus, being God himself, didn't need to inherit the grace of God from his Mother. God nonetheless chose to provide her with the graces to keep her sinless.