Your " scholar " is mistaken. You can go look for yourself. There is no NT material in the Dead Sea Scrolls.
As far as the Copts go:
Beliefs of Coptic Christians
The Coptic Church believes much of the same theology as other Christian denominations, including a belief in one God made up of Father, Son and Holy Spirit; that the Bible is the word of God; that Jesus took human form, died and rose again for the salvation of humanity; that Jesus will come again to judge both the living and the dead.
“The Lord Jesus Christ is God Himself, the Incarnate Logos Who took to Himself a perfect manhood. His Divine nature is one with his human nature yet without mingling, confusion or alteration; a complete Hypostatic Union…
… As this union is permanent, never divided nor separated, we say in the liturgy that His Godhead never departed from His manhood for a single moment nor even for a twinkle of an eye.” Pope Shenouda III, The Nature of Christ, 1999
Seems pretty mainstream Trinitarian Christian to me.
There were plenty of references to Trinity before Nicea and they had nothing to do with pleasing pagans ( were that the case it would have been better to have developed a Hindu-style being into which the pantheons of plural regional deities could have been folded as " emanations" . As it was, the Church Fathers took their thinking from concepts in Scripture and early church tradition) :
"Ignatius of Antioch (died 98/117). Bishop of Antioch.
"In Christ Jesus our Lord, by whom and with whom be glory and power to the Father with the Holy Spirit for ever" (n. 7; PG 5.988).
Justin Martyr ( 100-165)
For, in the name of God, the Father and Lord of the universe, and of our Savior Jesus Christ, and of the Holy Spirit, they then receive the washing with water" (First Apol., LXI).
Irenaeus ( c. 180) The Church … has received from the apostles and their disciples this faith in one God, the Father … in one Christ Jesus, the Son of God, who became incarnate for our salvation; and in the Holy Spirit. (Against Heresies I:10:1)
Tertullian ( c. 200) The simple, (indeed, I will not call them unwise and unlearned), who always constitute the majority of believers, are startled at the dispensation [of the Three], on the ground that the very rule of faith withdraws them from the world's plurality of gods to the one only true God. They do not understand that although he is the one only God, he must yet be believed in with his own order of things. (Against Praxeas 3)
Tertullian (160-215). African apologist and theologian.
"We define that there are two, the Father and the Son, and three with the Holy Spirit, and this number is made by the pattern of salvation . . . [which] brings about unity in trinity, interrelating the three, the Father, the Son, and the Holy Spirit. They are three, not in dignity, but in degree, not in substance but in form, not in power but in kind. They are of one substance and power, because there is one God from whom these degrees, forms and kinds devolve in the name of Father, Son and Holy Spirit." (Adv. Prax. 23; PL 2.156-7).
Origen (185-254). Alexandrian theologian.
"If anyone would say that the Word of God or the Wisdom of God had a beginning, let him beware lest he direct his impiety rather against the unbegotten Father, since he denies that he was always Father, and that he has always begotten the Word, and that he always had wisdom in all previous times or ages or whatever can be imagined in priority . . . There can be no more ancient title of almighty God than that of Father, and it is through the Son that he is Father" (De Princ. 1.2.; PG 11.132).
"For if [the Holy Spirit were not eternally as He is, and had received knowledge at some time and then became the Holy Spirit] this were the case, the Holy Spirit would never be reckoned in the unity of the Trinity, i.e., along with the unchangeable Father and His Son, unless He had always been the Holy Spirit " (de Principiis, 1.111.4)
"Moreover, nothing in the Trinity can be called greater or less, since the fountain of divinity alone contains all things by His word and reason, and by the Spirit of His mouth sanctifies all things which are worthy of sanctification . . . " (de Principii., I. iii. 7)
As you can see from above, Arthur Weigall ( what do you know of his actual religious beliefs?) was apparently not much of a church historian. Was better as a stage manager for shows, I hear. Catholics have fun with him. He thought everything in Christianity was of pagan origin, so I'm assuming that you, too, reject the virgin birth. He did. Would have loved to see his send-up of Islam.
There were plenty of pantheons of threes, twos, multiples all over the world. That does not mean the early Christians folded that in any more than you worship the moon. ( something plenty of detractors of Islam go to great lengths to prove)
I could fill this page with quotes of " scholars" of your own faith that are sure you are not Muslim but a corrupted form of
* eastern Christianity
* a mishmash of all of the above
I came on here to learn about you folks because whatever "Muslim scholar "wrote the only book on Islam we have in our tiny town library felt you were only worth a " they are of no importance" passage.
Am I supposed to believe them?