In a similar sense as when you do word search for "consecration of bishop" and declare it not scriptural. Indeed, veneration of saints is not referred to in the scripture for the simple reason that this form of piety developed in the age of martyrs after the books of the Canon were written. The necessary elements of the veneration of all saints, and of course, especially Mary are in the multiple requests for intercession throughout the gospel (e.g. see the daughter of Jairus episode), and in the fact that the saints have eternal life in Christ ("To them indeed, who according to patience in good work, seek glory and honour and incorruption, eternal life", Romans 2). To read more, see Communion of Saints.
As Catholics we do not think that our devotions and pieties need to be explicitly mentioned in the scripture. We also do not put too great a stock in pronouncements of unscripturality coming form the Protestants, since in their case the fundamentals of their theologies are not merely unscriptural but counterscriptural.
Veneration and devotions to the Blessed Mother come in addition from her essence as the Mother of God who adopted us all at the foot of the Cross. "All generations shall call me blessed", she predicted. We understand that she participated int he mystery of the Incarnation in a unique way. There is no reason to think that she would not wish to bring the divine plan of salvation to its most glorious fruition also now; and if so, certainly she will add her prayers to ours as we plead for Christ's mercy.
Specifically you refer to the mediatorship of Christ. You forget, however, that in the same passage where His role as the sole mediator is described, intercessory prayers are also prescribed:
1 I desire therefore, first of all, that supplications, prayers, intercessions, and thanksgivings be made for all men: 2 For kings, and for all that are in high station: that we may lead a quiet and a peaceable life in all piety and chastity. 3 For this is good and acceptable in the sight of God our Saviour, 4 Who will have all men to be saved, and to come to the knowledge of the truth. 5 For there is one God, and one mediator of God and men, the man Christ Jesus: 6 Who gave himself a redemption for all, a testimony in due times.(1 Timothy 2)
It should be clear that a prayer to Mary or to a saint is a prayer to Christ; but it is a stronger prayer as now two of us pray, me and her.
pray one for another, that you may be saved. For the continual prayer of a just man availeth much(James 5:16)
Your other specific objection is to consecrating the world to Mary, because "God alone is sovereign". Biblically, it is not a contradiction: as Christ is King, Mary is Queen Mother. Her last recorded word is "do as He tells you". We listen.
The Coronation of the Virgin
Gentile da Fabriano
The J. Paul Getty Museum