Has modern scholarship disproved the traditional authorship of the Gospel of John?

Of the four canonical gospels of the life of Christ, the one I have most often seen dismissed outright for historical value is the Gospel of John. The early church agreed that it was the latest written of the four. In the early church, the name attached to the gospel was that of John the Apostle. But scholars have found signs of editing; was it tampering? There is also clearly an appendix in Chapter 21 with multiple authors referring to themselves as "we" (John 21:24). Could anything refute the traditional attribution to John more clearly? Can anything in an altered document be trusted?

That depends very much on whether the author was aware of the editing process and whether he approved of it. Does the early church have anything to say about how such editing might have happened? As a matter of fact, it does. This excerpt is from the Muratorian Canon, probably dated to the late 100's A.D., commenting on how the fourth gospel came to be written:

When his fellow-disciples and bishops encouraged him, John said, “Fast along with me three days from today, and whatever may be revealed to each, let us relate it one to another.” The same night it was revealed to Andrew, one of the apostles, that John in his own name should write down everything and that they should all revise it. (from the Muratorian Canon, likely dates ranging from 170 A.D. – 200 A.D based on internal evidence. Emphasis added.)

The very early church, still in the 100's, retained this information on how the fourth gospel came to be written, how it came to be edited, and why it has an appendix. One of the names of the editors is retained for us: Andrew the apostle, who was Simon Peter's brother.

I would say that not only has modern scholarship not disproved the traditional authorship of the Gospel of John, I'd say that the Muratorian Canon's comments explain all the concerns that have been raised about the appendix and editing. The more interesting question to me is this: I'm curious whether modern scholarship has interacted with the Muratorian Canon's more detailed description of how the Gospel of John came to be written which would explain both editing and the appendix. I'm very curious whether modern scholarship has interacted seriously with the witness of the early church on how the fourth gospel came to be written.

For my own part, in the Muratorian Canon I see an explanation from the early Christian church that covers all the known facts and objections to traditional authorship. Unless a better explanation should be found, I will work under the view that this explanation is correct for the authorship and editing of the fourth gospel.