This causes me a great deal of problems. I agree that truth does not change. But what happens to those in previous ages who did not believe what was later defined? God requires that we believe certain things. What those things are is part of the truth. If the Church does not reveal what must be believed until a later period, then the Church has failed those who believed differently before the definition. I know the opinion that until definition it is just a theologumenon, but does God change what must be believed because the Church decided to define it? I can’t see that because the truth, including what we must believe, doesn’t change.
But to get to that promise you must agree that it means what the Church says it means. To arrive at that conclusion, you must privately interpret the evidence, biblical or otherwise, because until you do, how do you know that it gives the Church the authority it claims.
Based on what you are trying to prove, it would mean that all Jews before the Incarnation of Christ, and His Passion and Crucifixion were doomed to hell, because something else was revealed after their lives. All who followed the Laws of Abraham, and did not know the Mosaic Law, well too bad. For those who knew what was revealed to Noah, and not to Abram, who became Abraham, well tough kerplach. A very silly way of looking at things. We are responsible for what is given to us. If we look back historically, there was no need to define the belief in the Trinity until the issues of disbelief with Arius and Nestorius, If you asked someone if they believed in the Trinity prior to that time they would probably scratch their head, and wonder what you were talking about. So too when Truths that had been part of the common belief of Catholics prior to the rise of various errors were challenged the Church with it’s teaching authority, in order to combat the error defined issues clearly. Sometimes when the issues continued to rise up in various places, the Church had to confirm it’s previous statements.
As to your query, “…does God change what must be believed because the Church decided to define it? I can’t see that because the truth, including what we must believe, doesn’t change.” If you can’t see that, then it may be time to review Jesus words, “Whatsoever you bind in heaven is bound on earth, and whatsoever you bind on earth is bound in heaven.” He was speaking directly to the Apostles in one instance and to St. Peter in another.
Additionally, as we individually grow in our spiritual lives, and as the Church grows in understanding of God, it is not the Truths that change, but our understanding of the Truth. Some take this too far, as the Modernists of the late 19th through the 20th Century, and leave open the development of ideas that contradict the teachings held by the Church since the time of the Apostles.
With regard to the Immaculate Conception and the Assumption, these beliefs did not pop up out of nowhere, as many Protestants charge. I have a couple of altar Missals from the mid 1600’s, and have read Missals in the British Library while spending time in London, where the oldest manuscripts date from 300 BC, In the partial Missals from the 700’s the Feast of the Assumption and Immaculate Conception show up. As there is literary evidence that it pre-dates these missals, we can see that it has been a belief among Christians since very early.
If we look at history itself, we can see that Christians (and here I mean Catholics, since aside from a few small heretical groups like the Judaizers who died out for a time) in the first centuries had other issues they needed to focus on, such as staying alive, and during the outbreaks of persecution, handing on the infant Faith. We see the first apologetics defend the Faith against charges that the Church was Atheistic and Cannabalistic because Christians failed to worship the Roman Gods, and taught that the Eucharist was the Body and Blood of Christ. But there was little time to discuss more fully the truths that the Faith taught.
In the secular world we see the same ideas. The Protestant Minister and Scientist, Newton defined the laws of gravity. They existed prior to his definitions, but if you asked someone, “What are the laws that describe gravity?” they would give you a blank stare. Did the laws of gravity not exist prior to them being defined? Would you hold someone responsible for knowing the laws of gravity before they were defined?
As to your second assertion. Faith, which comes to us as a gift from God, by our accepting of His graces, allows us to make an assent to the Truths that Jesus taught. If as part of our acceptance of the evidence we believe Jesus was and is the Second Person of the Trinity, then we have to evaluate if He not only had the authority to apportion that authority to the Apostles, who we read in the NT are among those who are given authority over the Devil, and authority over others in the Church. You are free to reject what Jesus said, and follow any one of the Protestant Churches, or start your own. God will supply the Grace, it is up to you, (Unless you are a disciple of John Calvin) to accept or reject that Grace.
tough kerplach??? 
You can find part two in the list to the right of the video if you so feel inclined. 
98 cents & my opinion will buy you a Sweet Tea at Mickey D’s any day of the week).