How do I answer Protestants who cite Romans 3:23 ('all have sinned') and 1 Timothy 2:5 ('one mediator')?

It is annoying because they always bring out the same few verses; they have been trained to know them. This is called the Roman road, because most of them are in Romans, and they take advantage of the fact that many Catholics do not know the Bible. For a Catholic the greatest authority is the Church, so the Bible comes in second; a Protestant, who recognizes no head of the Church on earth, is left with only the Scripture, so he learns his verses. There is nothing wrong with quoting Romans 3:23, 'all have sinned and come short of the glory of God.' That is a fact: there is not one person except Jesus Christ and Mary. The Protestants do not believe Mary fills the bill, only Jesus, and that is one thing they hold against us, because we put Mary in that immaculate position of never having sinned and never having contracted original sin. Note your own distinction: someone can be sinless because they never sinned, and that is Mary, but Mary's sinlessness also depends on her never receiving original sin. Paul gives both reasons in Romans 5 for why we are sinners: we inherit a sinful nature from Adam, and we sin ourselves. So Luke is not saying Zachariah and Elizabeth were perfect people who never needed forgiveness. There was no confession in their day, but they could go to the priest with a sacrifice and be remitted of their sin and be considered just before God, as many Old Testament people were. Catholics were the first to teach that this goodness comes from grace, the power God gives us to please Him with good works; you can do nothing without the grace of God, but once you have it you have a great deal of work to do that never stops, which is why James can say Abraham was justified by works. As for 1 Timothy 2:5, you must read it in context. It comes right after 2:4, 'God desires all men to be saved and come to the knowledge of the truth,' one of the most mind-bending verses in the Bible, because if God is all-powerful, what holds back what He desires, unless men have the power to frustrate His will? So how does the salvation God desires happen? Only through Jesus Christ, the one mediator, the only one who died on the cross. Mary did not die on the cross; they even tried to make her Mediatrix of all graces and that failed. Paul is singling out Jesus as the one through whom salvation flows. The verse is not denying other things like the prayers of the saints; that is simply outside its context.