John 21:24 suggests that either the appendix or even the entire Gospel itself was not written by the hand of the "disciple whom Jesus loved.":
"This is the disciple who testifies of these things, and wrote these things; and we know that his testimony is true."
John 21:24 doesnt suggest that at ALL. In fact there is express testimony from the author himself:
“And he who hath seen hath testified, and his testimony is true, and he knows that he tells the truth, and he testifies so that you also may believe.” (John 19:35)
“It is generally inferred, probably rightly, that this witness is the beloved disciple (vv. 25-27), responsible for the fourth Gospel as a whole (21:24).” (D.A.Carson, page 625)
“By the pronoun 'ekeinos' (In the second half of the verse - 'He' knows that he tells the truth) the eyewitness refers to himself. This is certainly the easiest way to untangle the pronouns; ekeinos then resumes the referent in the preceding clause, “and 'his' testimony is true”. Certainly eikos can be used by an author about himself. In this case the most likely person is the beloved disciple himself - not only because he is in the vicinity (v.27) but also because this bears formal similarity to 21:24 where the beloved disciple is contextually identified.” (page 626, Carson)
In further emphasising the compelling connection between the evangelist, witness of 19:35, and the beloved disciples, Carson then draws a connection with 1:14: “we have beheld his glory”. “In the theology of the fourth Gospel, the glory of the Son is no more brilliantly displayed to a fallen world than in the shame and suffering of the cross. For the evangelist not to have been present at the supreme display of the Son's glory would be a betrayal of the anticipation called forth by the prologue. The theme of eye-witness testimony thus links not only 1:14 and 21:24, but 19:35 as well, especially since this is the hour for the Son of Man to be glorified.” (page 626, Carson)
Further information affirming that the beloved disciple is the author:
The beloved disciple, is found at the last supper (13:23), the cross (19:26-28), and at the empty tomb (20:2-9). In the epilogue he said to be the one who wrote “these things”, suggesting that the beloved disciple is in fact the evangelist of the Gospel. The synoptic Gospels insist that Christ's apostles were exclusive partakers with Christ at the last supper (Mk. 14:17), which means that the beloved disciple was one of the twelve apostles. The beloved disciple cannot be Peter, for he is repeatedly distinguished from Peter (John 13:23-24; 20:2-9; 21:20). Considering the fact he is one of the seven who go fishing in Chapter 21, and by implication, neither Peter, Thomas, nor Nathaniel, he is thus either one of the sons of Zebedee (James or John) or of the other two unamed disciples. He cannot be James, for James was the first apostle to be martyred (during the reign of Herod Agrippa I, A.D. 41-44; Acts 12:1-1), whilst John lived long enough to give weight to the rumour that he would not die until the Lord's return (21:23).
There is absolutely no evidence to suggest that the appendix was written by anyone other than the beloved disciple - and none whatsoever to deduce anything about the author of the Gospel body from anything written in the appendix. The opinion at present is fairly general, even among the critics - the vocabulary, the style, and the mode of presentation as a whole, plus the subject-matter of the actual passage reveal common authorship of this appendix and the preceding chapters of the Fourth Gospel.
The Fourth Gospel was a favorite of the Gnostics and and there was some difficulty in certain Orthodox circles to getting it accepted as apostolic.
This can be attributed to the fact that a) John's Gospel was written in a manner which made it difficult to understand, and b) It was often misused by Gnostics which would have instigated much cautiousness, discouraging a quick acceptance.
One could argue that we should believe the Gospel based on faith, not who may or may not have originally authored it.
If the Gospel in its current form were divinely inspired, it wouldn't matter who originally wrote it nor would it matter the date of its composition in its current form.
There is pleeeeeeeeeenty of sufficient external and internal evidence to support Johainne authorship. We shouldnt accept this Gospel's authenticity on blind faith, but on the clear early testiomny of the church - Is there really any good reason at all for discrediting Iraneus' quotation of Polycarp who expressly states that St John the apostle, the beloved disciple is the author of this Gospel?? NONE...all ive heard from anit-Johainne critics is speculation and conjecture - there is no good reason to accept the strong external testimony of the early church.
And if it were the writing down of an oral tradition that began with John, and if the writers of the Gospel were praying for the intercession of John while composing it, then they could have attributed it to him.
Ireneus states that John wrote the gospel to remove the error of Cerinthus but what if it were the disciples of John who edited the Gospel to remove its errors of Cerinthus?
What if it were even John himself using a Gnostic gospel as the primary source material, using his eye witness to expunge it of heresies and conform it to Orthodox doctrine?
Why do you choose to make all these conjectures, against the clear and strong external and internal evidence of Johainne authorship?