James 2:10 For whosoever shall keep the whole law, and yet offend in one point, he is guilty of all.And it gets worse. You know the Sermon on the Mount raised the stakes, so that overt murder and overt adultery are NOT the measure of sin, but any trace of lust, or any hint of anger without due cause, will bring the condemnation of violation of the law.
Romans 7:7-8 What shall we say then? Is the law sin? God forbid. Nay, I had not known sin, but by the law: for I had not known lust, except the law had said, Thou shalt not covet. (8) But sin, taking occasion by the commandment, wrought in me all manner of concupiscence. For without the law sin was dead.So we have it now that even dissatisfaction that leads to wrong desire was sufficient to condemn Paul, and James would further say that such a "victimless" sin, and seemingly so harmless, renders one a violator of God's entire law, and subject to all the horrific consequences stated therein. As someone who has defended clients under Illinois law, this is a defense I know in advance I could never win. God knows every secret passage in our hearts. No matter how outstanding we may appear to the world, all of us have transgressed some aspect of the law of God's perfect love.
I generally agree with your post. The purpose of St. Paul thickening the paint in Romans 3 is indeed to remind us that sin is sin, — at least if committed with full knowledge and willfully.
But the fact remains that “all have sinned” is in the context of “venom on the lips” and “feet quick to shed blood”, so unless you are prepared to advance the idea that Mary hastily murdered someone, you need to take the entire passage as a poetic exaggeration, including the “all” in it.