Your theory of a pre-existent Theotokos is unnecessary and frankly heretical, unless of course you belong to one of the sects which was a forerunner of Mohammedanism which held to a belief something like what you have said. But your comment does indicate to me that it is your Trinitarian theology which is deficient. Here's a simple definition to think on:
"Trinity is simple unity; it is not merged together - it is three in one. The One three-hypostatical God has the three hypostases perfectly distinct in Himself." +Gregory of Sinai
Now, tell me why we cannot call Mary the Theotokos unless we say that she pre-exists God?
If Mary is the Mother of God and mothers preexist their offspring then Mary preexists God. I don't see the problem for saying this since I have been accused of Nestorianism for declaring Mary is mother of Jesus.