Ah, let me see if I understand your view here. The entire orthodox Christian world knew nothing of the veneration of Mary.
Then in the 4th century, the entire ancient Church from India to Africa to Gaul suddenly realized "Golly gee! The Gnostics must be right!" and flipped over like a light switch.
And no one objected to this? Not one voice was raised against it? And just this *one* tenet of gnosticism was adopted, while the rest was despised as a heretical abomination from the pit of hell?
There is difference between prayers about, and concerning 'departed saints', and prayer directed TO departed saints, Mary included.
The latter (specific type of prayer) was what was generally missing prior to the 4th and 5th centuries, with it present first among Gnostics prior to then, and then somewhat famously among the Collyridrians who set out little cakes for Mary, "Queen of Heaven", and for doing so were judged to be heretical by Epiphanius of Salamis.
No, it was more like a subtle shift from veneration, to adoration, to prayer addressed directly towards "Mary" requesting her own personal intercessions.
Epiphanius did;
St. Ambrose;
At about the time of both those men was were things took off as for what can be rightfully referred to as 'cult of Mary'. St. Epiphanius, himself a devotee of Mary, serves as something of a way-point, a marker of sorts for what was more widely accepted among the learned, and what during his own era was considered to be going too far with it.
Prior to his own era, although there is trace enough for Mary being venerated, including being prayed about in gratitude and thankfulness to God for such as her and many others too, in addition to thankfulness to the Father for Jesus -- prayers directed to anyone but the One Eternal God is not among tradition as handed down by the chosen apostles. That is what I'm saying. Not something else, not some silly set-up as you've proposed.