First note that the concept of the Assumption is not foreign to scripture. We will all experience the resurrection of the Body when Jesus comes again (cf. John 5:28-29; 6:39-40; 11:24-25 and chapter 15 from 1 Corinthians to name a few). Likewise, Mary was not the only one to experience this before the second coming. Enoch (Gen 5:24; Heb 11:5) and Elijah (2 Kings 2:11-12; 1 Mac 2:58; Sirach 48:4,9) were assumed into heaven. Paul suggests that a third man may have been as well (2 Cor 12:2-3) and Matthew speaks of "many bodies of the saints" who were raised from the tomb after Jesus' resurrection (Mat 27:52-53). So, the principle is scripturally sound. We have left only to answer the question, "Did this happen to Mary?"
As far as explicit references go, this can be neither confirmed nor denied. I find nothing in scripture, explicit or otherwise, that condemns the teaching. What I do find, however, are several verses that seem to point to it, at least implicitly [from the RSV, unless otherwise noted]:
Psalm 45:9 daughters of kings are among your ladies of honor; at your right hand stands the queen in gold of Ophir.
Psa 132:8 Arise, O LORD, and go to thy resting place, thou and the ark of thy might.
Isa 60:13 The glory of Lebanon shall come to you, the cypress, the plane, and the pine, to beautify the place of my sanctuary; and I will make the place of my feet glorious.
So 3:6 What is that coming up from the wilderness, like a column of smoke, perfumed with myrrh and frankincense, with all the fragrant powders of the merchant?
So 8:5 Who is that coming up from the wilderness, leaning upon her beloved?
19 Then God's temple in heaven was opened, and the ark of his covenant was seen within his temple; and there were flashes of lightning, voices, peals of thunder, an earthquake, and heavy hail.
1 And a great portent appeared in heaven, a woman clothed with the sun, with the moon under her feet, and on her head a crown of twelve stars
Rev 12: 13-14
13 and when the dragon saw that he had been thrown down to the earth, he pursued the woman who had borne the male child.
14 But the woman was given the two wings of the great eagle that she might fly from the serpent nto the wilderness, to the place where she is to be nourished for a time, and times, and half a time.
I acknowledge full well that the literal intention of these verses is not to support the Assumption. However, even the NT writers themselves, when quoting the OT, were fond of seeing in various passages a meaning not originally intended by the author. The Messianic verses in particular are treated this way. Look at them (here). You'll notice that the majority of them were first about a king, or some holy object, or a prophet speaking about himself. But the NT writers saw them as pointing to Christ.
And so it is with these verses. Though, in the literal sense, they speak of other things, the Church sees in them certain forshadowings of Mary's Assumption. Why? Because just as the NT writers saw in the OT what they believed about Christ, the Church sees in the OT what she believes about Mary. The Church's beliefs come from the Sacred Deposit she has received, and the ECF's testify to this. The light of the NT and this Sacred Deposit often reveals meanings to Scripture other than what the authors initially intended.
Contrary to what you claim, modern research reveals evidence of belief in Mary's Assumption from as early as the third century. I offer you the following points:
What all of this shows is that scholars are beginning to find evidence of the assumption from a much earlier time then first expected. The fact that the earliest Marian feast in the Church is of her Dormition/Assumption (fourth century) tells us that belief in this was widespread and longstanding. Church feasts don't just sprout up with every "new" doctrine or "pious legend" that surfaces. They arise as a result of the traditional beliefs of the people.
From here we come to the more explicit references:
I had hoped to provide some more commentary on this evidence from Tradition, but since I have already exceeded my word limit, this must suffice. However, I anticipate being able to elaborate further on this in response to your analysis of it.
"If therefore it might come to pass before the power of your grace, it has appeared right to us your servants that, as you, having overcome death does reign in glory, so you should raise up the body of your mother and take her with you, rejoicing into heaven. Then said the Savior [Jesus]: 'Be it done according to your will" [Pseudo-Melito, The Passing of the Virgin 16:2-17; (300 AD)].
"If the Holy Virgin had died and was buried, her falling asleep would have been surrounded with honour, death would have found her pure, and her crown would have been a virginal one...Had she been martyred according to what is written: 'Thine own soul a sword shall pierce', then she would shine gloriously among the martyrs, and her holy body would have been declared blessed; for by her, did light come to the world" [Epiphanius, Panarion, 78:23 (A.D. 377), in PG 42:737].
"Therefore the Virgin is immortal to this day, seeing that he who had dwelt in her transported her to the regions of her assumption" [Timothy of Jerusalem, Homily on Simeon and Anna; (400 AD)].
"And from that time forth all knew that the spotless and precious body had been transferred to paradise" [John the Theologian, The Falling Asleep of Mary; (400 AD)].
"The Apostles took up her body on a bier and placed it in a tomb; and they guarded it, expecting the Lord to come. And behold, again the Lord stood by them; and the holy body having been received, He commanded that it be taken in a cloud into paradise: where now, rejoinedd to the soul, [Mary] rejoices with the Lord's chosen ones..." [Gregory of Tours, Eight Books of Miracles, 1:4; (575-593 A.D.) ].
"As the most glorious Mother of Christ, our Savior and God and the giver of life and immortality, has been endowed with life by him, she has received an eternal incorruptibility of the body together with him who has raised her up from the tomb and has taken her up to himself in a way known only to him." [Modestus of Jerusalem, Encomium in dormitionnem Sanctissimae Dominae nostrae Deiparae semperque Virginis Mariae (before A.D. 634), in PG 86-II,3306].
"It was fitting...that the most holy-body of Mary, God-bearing body, receptacle of God, divinized, incorruptible, illuminated by divine grace and full glory...should be entrusted to the earth for a little while and raised up to heaven in glory, with her soul pleasing to God." [Theoteknos of Livias, Homily on the Assumption; (before 650 A.D.)].