Wow, this gets stranger and stranger. Let me see if I understand this correctly; the Catholic Church declares something to be an infallible teaching of the RCC but this doesn't make it right for someone else???? Infalibility defined by the Church is:
It is well further to explain:
that it does not require holiness of life, much less imply impeccability in its organs; sinful and wicked men may be God's agents in defining infallibly;
and finally that the validity of the Divine guarantee is independent of the fallible arguments upon which a definitive decision may be based, and of the possibly unworthy human motives that in cases of strife may appear to have influenced the result. It is the definitive result itself, and it alone, that is guaranteed to be infallible, not the preliminary stages by which it is reached.
Infallible declarations do not exhaust every possible detail of a particular subject. For example, when Nicea in 325 declared that Jesus was "God from God, light from light, True God from True God", do you think that was the last time anyone discussed particular nuances of this relationship between God the Father and God the Son? No. Nicea laid out SOME of the future groundwork. The infallible teaching of the Immaculate Conception has laid the groundwork for future discussions about the Virgin Mary. They do not settle EVERYTHING, since it is the idea expressed by the words themselves that are infallible. Future generations may find that the written words used by the Pope were insufficiently nuanced to refute a future dilemna between Christians interpretating the Tradition expressed. Thus, as the Council of Ephesus and Chalcedon were required to solidify the relationship between Jesus and the Father, another Ecumenical Council will likely be necessary when we try to consolidate our respective traditions between the East and the West regarding Mary and being sinless.
So, if I understand you and the definition correctly, the Church can make an "inerrent" decision but it is perfectly acceptable for someone else to make another "inerrent" decision. In other words, there could be multiple truths. Is that's what you're stating?
No. There will not be a defined teaching that says "Mary is Immaculately Conceived" followed by a defined teaching that says "Mary is NOT Immaculately Conceived", for the Spirit guides the Church. It may become necessary to clarify the nuances of this doctrine, but that doesn't nullify what went before. The East does not dogmatically declare that "Mary was not immaculately conceived". What needs to occur is for both sides to verbalize their respective beliefs and traditions, with the Spirit gently molding the Council, so as to come to a further refinement of the Catholic faith.
On this subject, it very well may come to the Orthodox accepting the dogma, though they will certainly discuss the concept of original sin that calls into question the need for being immaculately conceived. However, since the doctrine is not solely based on original sin, I would say that it would be accepted by the East.
Regards