The western concept of original sin gets complex and confusing. This is unnecessary in Orthodoxy.
I do not think it does get complicated, and if it is confusing that may be an issue with preconceptions on the part of the listener. But in this case - that of Blessed Mary and the Lord Jesus Christ - it is fairly straight forward:
- Since the fall all humans who are naturally descended from Adam have a flaw in their human nature that is a consequence of the first sin.
- All human beings who are encompassed by the saving work of the Lord Jesus Christ will receive a restored and perfected human nature.
- The place in time in which any specific human being lives does not preclude the application of Christ's saving work to them.
Now, since the Lord Jesus Christ received his human nature from his mother, Blessed Mary, there is a question to ask;
Did Christ receive the flawed human nature that is the common lot of humanity born by natural generation?
If one says yes, then Christ was in need of salvation and hence of a saviour.
If one says no, then Christ was not in need of salvation.
Catholics say no, and reason that Christ needed no salvation is because he was sinless; he neither sinned himself nor was stained by original sin. And he was not stained by original sin because his humanity, received from his mother, was perfect. But it was innately perfect, not perfected by an act of singular grace whereby he was cleansed (saved). It was innately perfect because his mother's humanity was made perfect by a singular act of grace (saved) by the application to Blessed Mary of God's saving work in Jesus Christ, the Lord.
So the dogma of the Immaculate Conception is Christological, in that it is about the incarnation and the humanity of Christ.