What I really want is an answer to my question. Do the Old Testament verses Paul quotes in Romans 3 mean what Paul says they mean?
No, they don't. They provide a type, a reference point. All ADULTS, or nearly all (I can thing of severe cripples and quasi-vegetables to whom it may not apply) have committed sins under the Jewish law - but all adults are not UNDER the Jewish law. All adults, or nearly all, have also sinned under the law of Jesus.
Paul, and his followers, suggest that believing that Jesus was the Son of God is sufficient to be forgiven sins. But Jesus, the Son of God, said that to be forgiven one's sins, one had to forgive the sins of others against one's self.
Who is right? People who follow Paul are the ones who will say that by believing in Jesus they are saved, period. By this, I suppose they mean that they will pass final judgment and be sent into the City of God. They also believe that whoever does not believe in Jesus will be thrown into the Lake of Fire.
By contrast, those who think that Jesus has to be taken seriously will point to Jesus saying that he will say "I never knew you" to many who cry out "Lord! Lord!" to him, because "What good does it do you to say you follow me if you do not keep my commandments?"
Jesus said that each man will be judged by the standards of judgment by which he judged others. The forgiving man will be forgiven also, but the unforgiving man will be likewise not forgiven.
These are really two different religions, requiring two different things.
In general, the Catholic religion is one focused on Jesus, and noting that Jesus demanded that people DO things, and NOT DO other things. The Protestant religion, in general, is Pauline, in that it focuses on what a man believes, and says that if one believes in Jesus, everything is cool with God - indeed, they even say that Catholics saying that you have to DO the things that Jesus said you have to do is itself a sin, because it elevates "works", and "works" are unavailing.
That Protestant belief comes from Paul, and it is at the very heart of the difference between Protestants and the Catholic belief regarding deeds and acts - which are works. Catholics get their view from Jesus.
It is obvious to me that, given the conflict between Paul and Jesus, Jesus was God and Paul was not, so Jesus Trumps.
Protestants retort, relying on inerrant Sola Scriptura, that the entire Bible is of equal force, so what is written in Paul is of equal authority to whatever is written that Jesus said.
When someone like me replies that Paul and Jesus conflict, the Protestants reply "there is no conflict in the Bible" - and then they read it in a way that subordinates what Jesus said to what Paul said.
This is THE crux of the difference between Catholics and Protestants, theologically, though most don't realize it. Scripture is handled differently: to Catholics, the Gospels are the highest authority (that's why Catholics sit when the OT and NT epistles are read, but stand for the Gospel reading), because they contain the words of Jesus, and he was God Incarnate and Lord. To Protestants, there is no "highest authority" in Scripture: it's ALL God's word, ALL inerrant, and ALL of equal authority.
The net result is that Catholic doctrine quotes Jesus whenever there is a conflict in Scripture. Protestant doctrine says there is no conflict and quotes Paul for essentially everything.
These are two different religions, really, and you have put your finger, with your question, on THE Scriptural point of contention between the two.
Obviously I think the Catholics are right on the matter. I say Jesus trumps and that what a man must do is laid out as law by Jesus.
Protestants strongly disagree with this, because of the emphasis on DOING, which is "works", and they quote Paul.
Of course, I think Paul was talking about "mitzvot" under the Torah, specific ritual "works" under the Jewish law, and that "works" as HE meant the word is not the same thing as "deeds", "acts" and "works" in the context of the wider world. But I recognize that when I say this I am trying to save Paul from just being wrong.
I have also learned over the years that all discussions of this nature between Catholics and Protestants end in rancor and are for the most part useless. The Reformation was fought over this issue, and neither side persuaded the other, even by force, which is why there are two sides, and really, two separate religions - one based on doing what Jesus said, and the other based on believing who Jesus was.