The Bible is not a book of doctrines/dogmas. It has no list of approved teachings about any topic. To try to use it like that is to misapply and misinterpret what the authors of the various books were trying to convey to us. You must divest yourself of the “Sola Scriptura” mentality, which was one of the inventions of the reformers and not believed or taught up until they decided to use the Bible as it had never been used before. A great book, that you can read online on the topic:
Where We Got the Bible: Our Debt to the Catholic Church by Right Rev. Henry G. Graham. Granted, the bible has no “list”, but i believe 2Tim 3:16 would refute the above statement.
As to the verse you cited, if “everyone” is to be taken literally, it would mean that Jesus sinned, as well. Since that is ridiculous, we must ask: “What was St. Paul saying?” not “What does this verse say taken out of context?” Yes? St. Paul was discussing the grafting in of Gentile believers into Israel through Christ. He was saying that Jews and Gentiles alike are born into sin. He was not discussing Mary nor Jesus’s births or conceptions. Many non-Catholic groups cite this verse because is “seems” to say that Mary was born a sinner, but that is not what the text is discussing at all. There are many places in Scripture in which the word “all” is used in which is it not all encompassing, but rather refers only to certain persons/groups. This is where proper biblical exegesis needs to be applied.
Rom 3:23, Yes Paul was asserting both Jews/gentiles were sinners, and to take the verse literally would not mean that Christ was a sinner, as it seems the latter part of the verse “and come short of the glory of God”, is never taken into account. Christ was/is God, therefore He never sinned and couldn’t come short of His own glory! Scripture testifies that Christ was without sin, 2Cor 5:21, 1Pet 2:22, 1John 3:5, and concludes that all else where under sin Gal 3:22, Rom 5:12, Ps 14:3.
If you read Rom 3:9 onward, it is clear that indeed Paul is declaring all as sinners, (Christ excluded as ratified by scripture) and that verse 23, is not being taken “out of context” and a “proper biblical exegesis”, would confirm that “all” in this case means “all”!
Holy Ghost, inspired scripture, is not silent on the sinlessness of Christ, is not silent on the sinfullness of man, but is deafingly silent on the assumed sinlessness of Mary!!
I agree wholeheartedly that JW’s and other groups quote scripture out of context and would build a doctrine therein, but equally as speculative or maybe even more so is to build a doctrine or assert a dogma, that is without any scriptural support but would appear to be contrary to scripture. I make reference to " theologians and Bible scholars are free to speculate about any teaching/topic until the Church speaks definitely on it. This does not mean that the Church ever supported their speculations. The final say is with the Magisterium". In Acts 17:11, the Bereans did not automatically accept the teachings of Paul, but searched the scriptures for Authority.
Whilst i cannot be dogmatic that “brothers and sisters”, refers to the actual children of Mary, the greek word Adelphos, (brethren), used in Matt 12:46, Mk 3:31 and Luke 8:19, can mean countryman,half brother etc. It is also used to explicitly mean brothers from the same father/mother, Mat 4:18, Acts 7:13 etc. So to strictly apply the former rendition and build a doctrine around it, is fall into the very trap that you so rightly warned against!
Especially in the light of Matt 13: 55,56,57 and Mark 6:3,4, where the Greek word for sister, (Adelphe), is as far as i can ascertain used in reference to a family member/ direct sister. Exegetically, the weight of scripture would seem to point to Mary not being “ever virgin”.
When a person is in RCIA and knows little about Church teaching, it’s not a good idea to go to non-Catholic sources for information about Church teachings. The objections they raise have all been answered a million times over. You need to read competent Catholic sources.
Finally, everything the Church teaches about Mary is not to exalt her, per se. It may do that, but that’s not its primary function. Marian teachings, like all the Church’s teachings, tell us things about Christ, our place before God, and our duty to others. Certainly we love Mary, we ought to love all our brethren in Christ, non-Catholics, non-Christians, indeed, everyone. We honor her most especially for her fiat and her faithfulness. Think how hard it would have been for her if she had not been prepared by God for her very special mission as the Mother of God. Hers were sufferings and trials of the most unique kind–ones other mothers will never have to go through. Why? Because her Son was the Son of God and the Son of man. God prepared her for her special mission by preserving her from original sin, thus keeping her free of concupiscence (the desire to sin) because she was the Second Eve and the Ark of the New Covenant who conceived and bore in her womb the Christ of God.