"If she were made sinless from the moment of her conception, her holiness wouldn't be the supreme example of her humanity."
Eve was sinless from the moment of her conception/creation - are you saying that her humanity was somehow deficient because of this?
The contrast between Eve and Mary surely lies in the fact that both were sinless yet one disobeyed, and the other had to undo the knot of the earlier disobedience by her faith and obedience?
I don't know if being immortal is a "deficiency." Not knowing sin, and not having propensity to sin, Eve's holiness before disobeying was not an effort of any proportion; it was not any of her doing. Mary's holiness, by contrast, considering her mortality and human nature with propensity to sin but choosing not to, is a monumental achievement. There is simply no comparison. Comparing Mary to Eve dimnishes Theotokos.