I believe that you have it completely right here. There are a relatively small number of significant interpolations which have found their way into our Bibles, including John 5:4 (the stirring of the water at Bethesda), 1 John 5:7-8 (the Father, the Word and the Holy Spirit), Mark 16:9-20 (the late-added ending to Mark), Acts 8:37 (if you believe with all your heart…) and John 7:53-8:11 (the woman caught in adultery). This is the entire list of what I would call significant interpolations. Of the list, all but the John 8 interpolation are rejected by all or nearly all scholars as not being in the original and, therefore, not being inspired or authoritative. The exception is the story of the woman caught in adultery. This passage is found in the vast majority of manuscripts, but not in a small number of early manuscripts. What makes the case a bit confusing, potentially, is that the small number of manuscripts which do not include this passage here in John include the earliest and most authoritative manuscripts, such as Codex Sinaiticus and Codex Vaticanus, as well as two very early papyri. However, even when the story does not appear at John 7:53, it is found elsewhere–for example John 7:36, John 21:25 (at the end of John), or even at the end of Luke. Also, the story was being mentioned by church fathers all the way back in the second century.
My conclusion is that this was an extremely well-know story from the life of Jesus which was part of the oral tradition from the very beginning. It is possible, and perhaps even likely, that it was not found in the original gospel of John at John 7:53. It seems that the church as a whole was convinced that this was a legitimate story of an actual event which happened in the life of Jesus–in fact that it was one of the favorite of all the stories from the life of Jesus, but there was doubt about where it belonged in the gospels. For this reason, I have always assumed that this is a factual account of an actual event, and I, therefore, use it as an inspired account of an actual encounter of Jesus with the woman caught in adultery and the Jews who wanted to test Jesus. Personally, I do not hesitate to refer to John 7:53-8:11 as a true event in the life of Jesus. When I use it, I sometimes will inform my hearers that there is some doubt as to where it belongs in the gospels, but sometimes I do not do so, depending on the audience.
John Oakes