This is where the double standard of Protestantism comes in. Orthodox and heterodox Protestants accept the Catholic Church’s dogma of the Trinity, but reject the dogma of the Immaculate Conception and the doctrine of Mary’s intercessory role in the economy of salvation. If the Church is wrong in her Marian teachings, then how can Protestants be sure that the Church hasn’t erred in her Trinitarian and Christological doctrines? By arguing from the scriptures? If the scriptures are a sure guide, then how on earth have all these heresies arisen? And if there is no central Apostolic teaching authority of the Church or in Christendom, then who can know with certainty in his religious persuasion that his belief is the true one: baptism, the Eucharist, dispensationalism, marriage and divorce, etc?
First Mariology is not necessary to support Trinitarian doctrine. Secondly, the so called heresies you point were in some cases mere disagreements over fairly complex theological issues. For instance, Nestorius. I disagree with him, however, I certainly do not think it rose to the level of heresy. The same case with Origen. So frankly all protestants do not agree with everything labeled heretical by the ancient church. As a Calvinist certainly I would agree Pelasius’ doctrines were heretical, however, many protestants would disagree with me (and in fact the RCC and EOC has veered back toward his doctrines anyway). I mean seriously it took the church almost 800 years to finally agree it’s okay to venerate paintings and statutes, in clear violation of the second commandment.
The reformation occurred due to much more profound issues and Martin Luther was certainly justified in his rebellion. Since our schism the Catholic Church (the Roman Rite in particular) has embarked down a theological slippery slope, which IMO has been mostly due to her arrogance.
The immaculate conception (or the immaculate invention as I like to term it) is a 20th century innovation. Papal infallibility a 19th century innovation. These teachings enjoy no apostolic support – how do I know? Well for one they both came 1,900 years after the apostles were dead and buried (I might remind you all killed by the hands of Rome).