How do you know the first Christians did not believe any of those things? The writers from 100s-400s are the best we can go off of, as they lived in a time much closer to when the apostles were alive. Even if these beliefs came about through later revelation does not mean that they are wrong, either. The doctrine of the trinity didn't exist in the early church, but now almost all Christians believe in it because of the Council of Nicaea (325).
You cannot just attempt to read the Bible and hope that your interpretation is correct (are you studied in theology? Greek? Biblical Jewish tradition? Political happenings of the time?). This is why Christ founded the Church to help His flock. He didn't just give the apostles the Bible and say "Here ya go, good luck!" This is the confusion that has brought about the thousands of denominations we have today. Even if it were a good idea to just read the Bible and trust yourself to determine the correct Church, you would still have to trust the Catholic Church. The Bible as it is known today was compiled at the Council of Rome in 382 under Pope Damasus. Even if you believe that the King James Bible is the only one which can be trusted (which doesn't have the 7 books that were taken out of the Catholic Bible) you still need to trust all 66 books that were left that were originally compiled by Catholic councils.
There are no early church writings indicating that the early church believed that Mary was sinless, that we should pray to Mary, or that purgatory exists. Writings about that didn't come about until centuries after Christ. The Bible itself does not give any indication that early Christians believed in purgatory, sinlessness of Mary, or in the need to pray to Mary. Romans 3:23 says all have sinned and fallen short of the glory of God. Obviously "all have sinned" does not include Jesus because other verses say that Jesus never sinned. But there are no verses excepting Mary from Romans 3:23.
The Nicene Creed does not conflict with Scripture but is supported by Scripture. There were early church disagreements about the nature of the Trinity and they were resolved with the Council of Nicaea. I can accept the Nicene Creed because it can be derived from Scripture and is supported by Scripture. The belief that Mary never sinned conflicts with Scripture so I cannot accept that. We should check to see if something is consistent with Scripture and if it is not, we should reject it.
I agree that Christ founded the church. But how you do know what is "the church"? You are still exercising your own private personal interpretation when deciding what the church is. There's the Roman Catholic Church, the Russian Orthodox Church, the Greek Orthodox Church, the Oriental Orthodox Church, the Syrian church, the Ukrainian Orthodox Church, the Ukrainian Catholic Church, Coptic Churches, Monophysite churches, the Armenian church, Nestorian Churches, Eastern Rite Catholic churches, Sedevacantist Catholic churches, etc... How you do know which of these you are supposed to choose? You still must fall back on your own personal judgment.
And then even within these religious traditions you have divergence of belief--within Catholicism you have Franciscans, Jesuits, Benedictines, Cistercians, Augustinians, Catholics who favor women's ordination, Catholics who favor liberation theology, etc.... There are about a billion Catholics in the world and each one may have his own personal spin on things. Protestants have disagreements but they are united in the belief of Sola Scriptura, that the Bible is the ultimate source of authority.
The Catholic church approved a Bible that had already become generally accepted. The Bible did not come about as a result of the Roman Catholic church.