My specific difficulty is her being without sin. I can accept the other beliefs. If she was without sin - then what pupose did Christ serve? God sent His Son - to live a sinless life - by doing that do be the perfect sacrifice by which we may all partake in the gift of eternal life. If Mary was sinless - she could have done that and therefore Christ was not necessary.
She was saved by Christ outside of time. She didn’t remain sinless under her own power; it was by God’s grace, through Christ, that she was saved from sin,
because Christ was going to die on the Cross.
Is there any evidence that the early christian fathers believed in her sinlessness?
Yes; they were continually referring to her as “the new Eve,” which means that they understood that Mary, like Eve (and unlike every other woman ever born), was conceived without sin.
When you ask yourself, "Why wasn’t she thought of as “the new Rebecca,” who was the mother of Israel, or “the new Hannah,” mother of Samuel, etc., who were great because of the great men to whom they gave birth, then you realize that there has to be some quality about Eve herself, and not just to whom she was the mother, that makes Mary uniquely important.
But it was because of Christ that Mary received this great gift, and it was through Christ, though outside of time, that she received it; she didn’t gain it by her own efforts.
