Was 'Jesus and the woman taken in adultery' in St. John a later addition, and does this invalidate infallibility?

I believe that if any part of the Bible was changed in a way that matters, God would make sure the oldest complete New Testament we have, the Codex Vaticanus (note: Sinaiticus could be older but has missing books and added books and unlike Vaticanus wasn't necessarily always with the church) would have the right reading.

The Codex Vaticanus does not include this passage and the passage is as such not to be viewed in higher regard than the Didache or works by an elder in the 5th century.

I suspect the early church fathers found no provocative revelation in this addition, if it is indeed an addition, to have concern about inclusion. Mark 16:9-20 seems to have been questioned early on as to its accuracy as part of the original text. Jerome states he had early sources without these verses; conversely, Irenaeus is found quoting from these verses (16:19). As I read it, I judge little inconsistence in the teaching of Jesus, or his responses to legal practitioners and the Pharisees. It might have gotten less attention had it been included in the Book of Matthew rather than Book of John considering the agenda of the Matthew sect (overtly confrontational with the Pharisees being the norm).

Of the first scenario, John himself added this passage later on:

The earliest surviving manuscript to contain the Pericope Adulterae is from the late fourth or early fifth century, although there are hints that the pericope was known already in the fourth century. There is little doubt the Gospel existed for centuries without this passage, so we can rule out the possibility of John having added the passage himself, as an afterthought.

Of the second scenario, John told this story enough that later his followers thought it appropriate to add this passage to John’s account:

Whoever added this passage lived centuries after the original author and could not have known what he talked about.

Jumping forward to the fourth scenario, John himself had written this account elsewhere, and his followers later combined it with the rest of his Gospel:

We know that John's Gospel was originally anonymous, and it appears that it was attributed to the apostle John around 180 CE, quite a few decades after the gospel was written. We do not know who the author really was, so even if he did write other related material, it is improbable that Christians would have known the manuscripts to have the same source, especially after a period of centuries.

This leaves us the third scenario, This account was so common among early Christians and fit so well at this point in John’s gospel that the early Christians felt is appropriate to add this story here:

It is not necessary for the pericope to be in common usage for early Christians to feel is appropriate to add this story somewhere in the gospel accounts. John 7:53-8:11 is likely to have been the location where it was first inserted, but some manuscripts have it elsewhere in John's Gospel. Clare K. Rothschild notes in Paul in Athens, page 104, that it was also at one stage added in Luke's Gospel, one place being after Luke 21:38 and another being after Luke 24:53.

Does it invalidate biblical infallibility?

Only to the extent that it causes us to recognise that not everything we read in the Bible was there from the beginning. It could be said that God inspired others to add material with moral or religious application, although it could also be asked why God did not so inspire the original authors. But even if the passage is not truly inspired, it has the same moral and religious applications today.

Forgive me for answering my own question…

There are several scenarios I can think of regarding this passage.

1. John himself added this passage later on.
2. John told this story enough that later his followers thought it appropriate to add this passage to John’s account.
3. This account was so common among early Christians and fit so well at this point in John’s gospel that the early Christians felt is appropriate to add this story here.
4. John himself had written this account elsewhere, and his followers later combined it with the rest of his Gospel.

If any of these explanations are true, or something similar happened, it would not invalidate the truth of the account historically, or otherwise. Likewise, just because the very earliest manuscripts do not contain these words exactly it does not necessarily follow that it is not true or any less inspired.