All or nothing was probably a consideration. The first attempt (or, at least, noteworthy one) at creating a Christian canon was a gnostic attempt which was basically Luke, Acts, and Paul's letters (this is called Marcion's New Testament or canon). But this canon also cut out parts of Luke that made Jesus seem to Jewish. In comparison, a criteria people use when talking about the Gospels is the "criteria of embarrassment," which is a note that the Gospels preserve sayings and accounts that would actually give others cause to discredit them (like women being the eyewitnesses at the empty tomb). I don't think there's any real concern to think overt and deliberate editing is something of a concern in the New Testament's canonization. Now, for sure documents changed over time, not all copyists are good copyists and different geographic areas tended to approach the text differently. But I am neither a textual scholar nor an expert on the process. But as a MA student in theology, I hope I can address another part of your question.

The Gospel of Thomas is not in the canon because the early Church did not regard it as authoritative. It is associated with some form of gnostic thought and thus not seen as apostolic. As potential eyewitness, or near eyewitness, account it didn't seem to convince the streams of Christianity that would become orthodox. There seem to be documents referring to Thomas by Church authorities that are dismissing it outright. It's not canon because it was never widely seen as fulfilling the criteria of being apostolic (originating from an apostle, or someone close to an apostle) or catholic (which pertains to its widespread usage). The church of the first few centuries created a criteria for what was valid and what was not in an attempt to ensure that only writings in continuity with the apostles (who were assumed to be in continuity with Christ) were preserved. It's a criteria that was created expressly to discredit works from dissident sects. [Added: I would say at the heart of this concern is to preserve something akin to an accurate, eyewitness account -- or the closest they had -- so historical accuracy was, I think, a concern]

For our own consideration, I don't believe the arguments to trust Thomas meet the standards that most of the New Testament writings do. I tend to side with those that date it later because I think the arguments to favour it as a first-century writing are not methodologically sound. Thomas is commonly validated by authors on the basis of it's similarity to Mark and Q. If Thomas is close to Mark and Q, it must be an early document. The issue is that the Q document is a hypothetical document (it's supposedly a collection of sayings of Jesus, not a narrative) that was used by the writers of Mark, Matthew, and Luke (a common source theory is Q predates the Gospels, Mark is the first Gospel; Matthew and Luke are later Gospels that use Q and Mark as sources). Many people who date Thomas as an earlier writing are also interested in reconstructing Q as a way to try to prove which sayings of Jesus are authentic and which are not. It is not uncommon for argument for Thomas' trustworthiness to circle back to include an affirmation of Q, which was always and explicitly hypothetical document coming out of German scholarship during the Enlightenment. It's a very questionable historical enterprise, in my opinion.

Thomas might very well be a great insight into what gnostic sects actually taught, how they viewed Jesus, and how they viewed the dogma of the day (and, thus, their own dogma). It is definitely an important document that helps us understand the nature of the arguments in early Christianity.

Mark Allen Powell, in Jesus as a Figure in History, deals with this in his chapter titled "Sources and Criteria" and "The Jesus Seminar." Thomas R. Yoder Neufeld also speaks about this in his Recovering Jesus: The Witness of the New Testament, in particular in his chapter "Development of Jesus Traditions."