So are you saying that a linguist as impressive as Jerome mistranslated the term?
It seems more realistic that he translated the phrase to mean EXACTLY what it does mean. It's the inconvenience of easily accessible Scriptures, and an astoundingly more literate populace that provide a more probable explanation for the RCC's need to "adapt" the meaning of this term.
Throughout history, and especially during the 4th and 5th centuries, the basic category for thinking about Mary was that of paradox: Virgin and Mother; Human Mother of One who is God, Theotokos. For the most comprehensive--and in the opinion of many the most problematic--of all the terms invented for Mary was...Theotokos. It did not mean simply "mother of God," as it was usually rendered in Western languages (Mater Dei,and then in the Romance languages, or Mutter Gottes in German) but more precisely and fully "the one who gave birth to the one who is God" Therefore Bogorodica in Russia and its cognates in the other Slavic languages, and more seldom and more precisely. Deipara even in Latin).... It seems to have been a word of Christian coinage, not as some would have it, an adaption...of a name originally given to a pagan goddess. (Mary through the Ages, p. 55)
He goes on to quote John of Damascus as summarizing the Orthodox case for this title. Hence it is with justice and truth that we call holy Mary Theotokos. For this name embraces the whole mystery of the divine dispensation. For if she who bore him is the Theotokos, assuredly he who was born of her is God and likewise man...The name in truth signifies the one subsistence and the two natures and the two modes of generation of Our Lord Jesus Christ.( ibid, p.57.)
In other words, the title is really about who Jesus Christ is.