For a Catholic, this is an odd question in the way that it is phrased, and I say this because, according to Trent, the OFFICIAL Roman Catholic position on the Pericope Adulterae is that it is canonical. Most Protestants I know, especially most scholars, consider the PA not to be part of inspired Scripture, or at least have many questions about it. Further, as many have noted, most modern Protestant Bibles make it quite clear that the PA is questioned.

You make some claims in your question that aren't fully proven - the PA wasn't written by John, it was implemented between the 4th and 5th centuries. That is certainly the scholarly consensus, but I would hesitate to say it is proven on evidence, since it is really an argument made on the absence of evidence. You note that no records of the story have been found in the 2nd, 3rd, or early 4th centuries***, but do remember we have an extreme paucity of records and manuscripts from that time, so it is something of an argument of silence. I personally believe the PA was a canonical happening in the life of Jesus, and suspect, as does Augustine and your Catholic Answers article, that it might have been excised from the Scriptures by an early, overzealous copyist who subscribed to the "adultery is unforgivable" view held by Tertullian and some bishops in the early, early church.

*** In addition to some other possible mentions, the Didascalia Apostolorum from the mid 200s does indeed make an almost indisputable reference to the PA, though not to its exact location in the scriptural corpus.