The Catholic understanding of Grace is that it displaces sin, not merely covers it up. This is why one described as filled with grace no longer has sin.
Further, the Catholic Church does not teach that Mary and Jesus alone were without sin, and that all the rest have committed sins at least once. It is entirely compatible with Catholicism to think that John the Baptist was also without sin. In the case of Noah, for example, the Scripture tells us that he was “perfect in every way”, so even based on the Scripture alone you are required to believe that Noah was without sin.
So, you are setting up a straw man. You presume that we teach that St. Stephen was not in fact without sin as he was martyred. But we don’t teach that. In fact, we teach that everyone who validly received the Holy Communion is likewise free from sin from that moment on till he either dies or commits a sin. Baptism, ditto, frees one from sin. You probably disagree with all or with some of these doctrines, but you cannot say that juxtaposing St. Stephen being “pleres charis” and Our Lady being “kecharitomene” (the underlying Greek is in fact different) you are pointing to some contradiction in Catholic teaching.
Having said that, let us examine the contexts. Our Lady is proclaimed by Archangel Gabriel already filled with grace. This is why the grammatical prefect tense is important: here is a young girl and she is said to be filled with grace already. This the scriptural basis not merely of her sinlessness but also of her immaculate conception: since she had been filled with grace prior to Archangel Gabriel talking to her, it is reasonable to think that she had been that way since the beginning of the life, since Sts Joachim and Anna concieved her.
No similar inference exists with St. Stephen. He is undergoing martyrdom, and martyrdom is like baptism. He is filled with grace at that moment, but nothing can be inferred about his condition prior to that from that verse in Acts.
This is the fundamental leap of faith ex scriptura that most have a problem with (including the Orthodox churches, according to Kolokotronis) - being filled with grace does make one blameless and washes away your sins, but it does not imply that you were in that state since birth.
So, you are setting up a straw man. You presume that we teach that St. Stephen was not in fact without sin as he was martyred.
I did no such thing, and if it was implied, then accept my apologies! Rather, I wanted to use the example of Stephen being filled with grace as a case where a clearly NOT-sinless man was also filled with grace, as Mary. Meaning that being filled with grace does NOT confer with it a state of being without sin since conception. In effect, Stephen is a key example that shows the opposite of the conclusion made about Mary.
Our Lady is proclaimed by Archangel Gabriel already filled with grace. This is why the grammatical prefect tense is important: here is a young girl and she is said to be filled with grace already. This the scriptural basis not merely of her sinlessness but also of her immaculate conception: since she had been filled with grace prior to Archangel Gabriel talking to her, it is reasonable to think that she had been that way since the beginning of the life, since Sts Joachim and Anna concieved her.
Except that the root caritow means favored, NOT sinless. In fact, there is no foundational claim for caritow to mean sinless - either canonically or implicitly. Mary was favored, and found favor in the eyes of God, but she was NOT sinless. Perhaps you could try to interpet the Bible in that way, but it is far from obvious or unequivocal.
Furthermore, one needs to look no further than Romans 3:23 - ALL have sinned and fall short of the Glory of God. Not all but Mary, but ALL. Man's sinful nature is complete and inherent since Adam and Eve.