"If Mary was without sin as some believe, then Jesus could not be her Savior because she would have had nothing to be saved from."
There are two ways to save somebody from a pit: by pulling them out, or by preventing them from falling into a pit they otherwise would have fallen into.
Mary was saved the second way, in view of her being destined to be the Mother of the Messiah: she was to be the unique and only source of His human nature (since He had only one human parent), so the human nature she passed on to him would have to be complete and unstained by corruption: unwarped by iniquity and the curse.
I can't think of another logical way to interpret the title the Archangel revealed in his greeting: Kecharitomene.
When an angel appeared to Isaiah, it was basically,"Yeah, you DO have unclean lips. If you're going to speak God's words, you need radical cleansing, you need to be purified with a burning coal." But when the Archangel came to Mary to bear the very Word of God within the intimacy of her body, building Himself literally from her substance, there was no hint of such a purification being needed. Just the opposite, he said she was Kecharitomene: the lady who has (already) been filled with grace.
"Throughout history, God has selected mortal, sinful men to carry out his plan."
Exactly true.
"Mary was not a goddess, and needed the same grace for her salvation that we require." Exactly true again. She needed grace. We believe she received it when she needed it: at the beginning of her existence, Day One of her prenatal life.
Pure GUESSWORK on the part of Rome.
('cept Mary of course!)