The original texts of the biblical books were inerrant and inspired 100%, not just not the preservation, right? For example, in the case of Kings, Chronicles, Samuel, Nehemiah, Ezra, Gospels, etc. we have more than one account of the same info, so we can compare them against one another, and this is where some of the scribal errors seem to crop up. However, the majority of the Bible has no corresponding text to compare against for potential transcription errors. So if we have evidence that numeric and maybe other transcription errors occurred, how can we be certain that no doctrinal errors occurred elsewhere in the Bible? How would we know if they had occurred if we don’t have other texts to compare against? I know we can point to a unified message of the Bible as evidence, as well as earlier copies in addition to later copies. Do we have any other evidence that all doctrinal portions of the Bible were preserved?
Yes, I believe the originals were perfect. Yet even then that isn't the point. The point is that God’s word (his message to us) has been preserved, though not necessarily the container in which it was placed, if you get my meaning. See the relevant chapters of Compelling Evidence, or listen to How We Got the Bible, for more of my thinking.
And you are right, the originals, in the absence of parallel texts, would have no proof of perfection. But we do have multiple mss for each book of the Bible.
Finally, most importantly, no doctrine has ever hinged on the presence of a textual variant.