i Think i have best explanation when it comes to gospel of barnbas.
First, gospel of barnbas is not a book of God, it is barnabas memories written down in a book. So if a person write stuff from the memory, of course he will make mistakes, so those mistakes in barnabas gospel is proof that it is not forgery but actually proof that it is not forgery, beacuse if someone claim it was muslims who wrote it, then muslims cant lie, that is terrible sin, second if they wanted to match with islam they would not do such obvious errors.
So errors could arrive when person try to remember what he experienced long ago, and then start to write, and it is in human nature to make errors.
221: Jesus Chargeth Barnabas to Write
Jesus turned himself to him who writes, and said: "Barnabas, see that by all means you write my gospel concerning all that has happened through my dwelling in the world. And write in a similar manner that which has befallen Judas, in order that the faithful may be undeceived, and every one may believe the truth." Then answered he who writes: "I will do so, if God wills, O Master; but I do not know what happened to Judas, for I did not see it."
Jesus answered: "Here are John and Peter who saw everything, and they will tell you all that has passed." And then Jesus commanded us to call his faithful disciples [so] that they might see him. So James and John called together the seven disciples with Nicodemus and Joseph, and many others of the seventy-two, and they ate with Jesus.
The third day Jesus said: "Go to the Mount of Olives with my mother, for there I will ascend again to heaven, and you will see who shall bear me up." So they all went there except twenty-five of the seventy-two disciples, who for fear had fled to Damascus. And as they all stood in prayer, at midday Jesus came with a great multitude of angels who were praising God: and the splendour of his face made them greatly afraid and they fell with their faces to the ground. But Jesus lifted them up, comforting them, and saying: "Do not be afraid, I am your master."
And he reproved many who believed that he had died and risen again, saying: "Do you hold me and God for liars? I said to you that God has granted to me to live almost to the end of the world. Truly I say to you, I did not die; it was Judas the traitor. Beware, for Satan will make every effort to deceive you. Be my witnesses in Israel, and throughout the world, of all things that you have heard and seen."
And having said this, he prayed God for the salvation of the faithful, and the conversion of sinners and [then], his prayer ended, he embraced his mother, saying: "Peace be to you, my mother. Rest in God who created you and me." And having said this, he turned to his disciples, saying: "May God's grace and mercy be with you." Then before their eyes the four angels carried him up into heaven.
so barnbas wrote this book according to his memories and what other discples told him and remember to have happened.
and we all know, people can't remember Everything, that is why errors could have emergaed in this bible.
For me, at first errors proves this is not forgery, beacuse no muslim would ever say muhammed is messiah and jesus not messiah, or other stuff which contradict qur'an.