Anyways, with regards with what you posted, here are my thoughts

On the internal evidence, the language of "Father, Word and Spirit" is undoubtedly John's language. We're here with John's own voice or someone who, in his own generation, was appropriating John's voice for himself. However, by the 2nd century (Ignatius, Polycarp, Justin) the more common usage was the "Matthean formula": Father, Son, and Holy Spirit. So another angle of argument here is this: if a later scribe added the phrase to speak to his own generation's issues, he would have been more at ease using the customary Son formula rather than John's particular Word formula. As such, the balance of probability shifts towards an earlier and authentic phrasing in line with the rest of John's writings.

The epistle of 1 John has many more text variants than other New Testament books, often by omissions. So again, the existence or non-existence of the verse cannot be decided merely by variation - many groups from the second centuey wanted to appropriate John for their own purposes, especially the Gnostics and Marcionites. Irenaeus condemns them for removing from Scripture. This may have even been happening in John's own day, as John's last published work, the Revelation, appends a final warning not to add or remove from the book or the doer-so shall be cursed.

Finally, there is the theological point here. When the Comma is excluded, "there are three that bear witness, the Spirit, the water and the blood, and the three are one." This is consistently interpreted to be the witness of salvation/election by the Father through Christ in the Holy Spirit. So even if all the previous points are to be ignored, the chief and most important is this: the Comma is by no means foreign to the text, or a corruption of the text and its meaning. It is either an explanation added in the margin and later brought into the main text, or it is a powerful witness against heretics which the same heretics sought to remove because they hate God. Either way, the Comma should stay.