From my understanding, Maryology emerged in the centuries following the Early Church from the thorny issue of how Jesus could be sinless if He was born from the seed of Adam. Obviously since Joseph was His earthly father that's not a problem; so now we turn to His earthly mother, Mary. Ascending her into a divine realm as Mother of God seemed to fix the problem nicely. However, I would point out that you're still left with the issue that the Messiah had to be a descendant of David (and this couldn't be if He had no human parents).
Yet when many Catholics are faced with zero testimony for one of their beliefs, such as the Assumption or prayer to created beings in Heaven (which among other things means such have the power to hear prayer in Heaven, which only God is shown to have) then they God argue that nothing is impossible with God.
And thus since God is said to have preserved Mary from sin without her own parents being so, then they should not require her to be sinless in order for the sinless Christ to be born of her.
And if being sinless was required in order to bear the sinless Son of God, then why would she also not have to be perfect in character in order to bear one who was? But of course, for many uncensored Catholics, pretty much whatever Christ was and is, Mary also was and is , except as possessing Divinity by nature and perhaps headship over Christ.
Meanwhile, God "brought forth" His pure word thru imperfect but holy men, and as you point out, The sinless Savior had a sinful genealogy, coming out of Israel according to the flesh.