Jesus died for man and the Bible was written for mankind. The Scripture that all have sinned would apply to mankind.
“then you are making an appeal outside of the text”
I don’t see anywhere in the text that says Mary was sinless. If she needed to be sinless so that Jesus could be sinless didn’t her parents need to be sinless for her to be sinless and so on?
I really do not understand.
Mary did not need to be sinless so that Jesus could be sinless. Jesus is God and therefore sinless.
Mary, as the Ark of the New Covenant needed to be pure in order to hold the Word of God.
I do not think that anyone says that Jesus's sinlessness was hereditary or that God "could not" (though when one discusses a once in all of history event, it's hard to distinguish between what God COULD have done and what He DID do) have produced His incarnate Son from a fallen woman -- or out of clay.
This is by no means authoritative, but my impression is that the language used is "fitting." If one believes it, (as I do, leaving the whole hymen issue out of it) then it is just plain "nice" that the 9 month long "ark" which bore the Word was as morally and spiritually beautiful as the ark which bore the Torah was physically and sensibly beautiful.
I don't think "need" really enters into it.
BTW, bless you for your gentle inquiries. I hope my replies are half as gentle.