First of all, it is not “RC tradition” the establishes the doctrine that Mary ought be called Theotokos, which is usually Englished as “the Mother of God” or “the Birthgiver of God”. The Latin church is not the only Christian confession to acknowledge the authority of the Third Ecumenical Council. We Orthodox Christians regard its Acta (along with those of all seven — some of us would argue nine, though we differ from the Latins on which councils count as Eight and Nine — Ecumenical Councils) as authoritative statements of Christian belief, as do the Monophysites (Copts, Jacobites, and Armenians).
There is, of course, an ancient church which objected to the Third Ecumenical Council and went into schism over the issue: the Holy Catholic Apostolic Assyrian Church of the East, to give its full self-proclaimed name, which venerates the heretics Nestorius and Theodore of Mopsuestia as saints, and had converted a great part of Asia to Nestorian Christianity before its near-destruction by the Muslims. Although in theory their patriarch is the bishop of a see in Iraq, he actually resides and has his functional cathedral in Chicago.
Second, are you really so scrupulous as to never use technical language derived from the plain meaning of Scripture, but not used in Scripture? Do you never speak of “the Holy Trinity” as short-hand for “Father, Son and Holy Spirit, One God”? Are all the technical words your confession uses really vetted as sound translations of words in the Greek (or Hebrew since I suppose your confession favors the Masorete over the Septuagint) versions of the Scriptures?
Third, the point of the title is christological: Jesus Christ is God, fully and completely, just as the Father and the Holy Spirit are God (though there is but one God). He assumed our nature, fully and completely, and was born of the Virgin Mary. She is His mother, and by that fact, that she is His mother, He assumes our nature and all that pertains to it (excepting sin). To deny her the title “Mother of God” one must split the person of Christ into “the Divine Logos and the One From the Virgin” as old Nestorius did, or deny the reality of Christ’s divinity as the Muslims do or of His humanity as the docetists and gnostics did. It’s not about Mary, it’s about Jesus. If you get it wrong, you get wrong who Jesus is.
Of course, if you discount the authority of Holy Tradition, of the Ecumenical Councils and of the local councils whose Acta they adopted, you have a real problem: what constitutes your Bible, which books are Scripture or are not Scripture, is itself a matter of Holy Tradition, set down first by a local council held in Carthage (the Latins regard this as having fixed the canon, since Carthage was in the Patriarchate of Rome and the Pope of Rome accepted its Acta) and made binding on the Church by the action of the Sixth Ecumenical Council (some would argue the Fourth on the basis of a canon ratifying “the ancient canons”, but this is obscure and the disciplinary session of the Sixth Ecumenical Council,
called in the west the Trullan Synod or Quinsext Council, explicitly gave the Carthaginian canons, including the
list of the books of Christian Scripture, ecumenical force.) Or, rather, if, as I suspect, you favor Luther’s abridgement of the canon, tossing out books St. Jerome erroneously thought of less importance because the Christ-denying Jews had dropped them from their canon, or mere human tradition.
It is no coincedence that Islam’s teaching about Christ developed where the Nestorian heresy flourished.
The Orthodox view of Tradition isn’t a mathematical equation, but expresses the broader view of the entire Church, not just a few theologians.
The Quinisext Council was even regarded by Roman Catholic canonists such as St. Francis De Sales as ecumenical.
And first besides those of the first rank whether of the new or of the Old Testament, about the year 364 they were received at the Council of Laodicea, Canon lx., (which was afterwards approved in the sixth general Council *),
http://www.goodcatholicbooks.org/francis/catholic-controversy/protestant-scripture.html#CHAPTER_III
So the Council of Trent was not the first Ecumenical Council to touch on the place of the disputed books in the canon.
If the opponents of those books are consistent, and I don’t think they are, then shouldn’t they toss 2,3 John, James, and Revelation out of their Bibles. Not to mention the Book of Esther, which St. Gregory the Theologian didn’t consider canonical.
If the Church has the power to reverse earlier decisions against their inspiration, then why doesn’t it have the authority to discover that the books of the Maccabees, Judith, etc. are canonical and inspired?
If they are logically consistent, then they should concede that their attack against the Roman patriarchate as the “Whore of Babylon” is rooted in an apocryphal book? Not to mention that the attack on the Divine Liturgy as a sacrificial banquet, considering many considered Hebrews apocryphal.