Earnest:
So the central question which no-one seems to want to answer is, if scribe A (or the scribe of his exemplar) did not understand Revelation 3:14 meant Jesus was God's first creation, why did he alter it?
If you are referring to the Codex Sinaiticus, this is an answer. The Codex Sinaiticus, and the book of Revelation found therein, is not the same text as what came to be accepted by Christianity and the Church almost a generation later.
The entire Codex not only contains a different reading at Revelation 3:14 but in many other areas throughout, not only Revelation but in every one of its books.
Not only that, it has other books in its collection not found in the New Testament canon--because there was no official canon yet. When the Codex was assembled, the Marcionist threat was a problem for Christianity (the teaching of salvation limited to the means by learning from a canon assembled by Marcion of Sinope, a heretical bishop that claimed he was "a follower of Paul," the apostle).
The Codex contains in its canon not merely variations, but two of the most popular books of Christianity: The Epistle of Barnabas and part of The Shepherd of Hermas.
This Codex was created before the canonization of the New Testament. It is generally understood to be from the family of works commissioned by Roman Emperor Constantine after his conversion to Christianity in 312 CE.
The New Testament canon's list of 27 books was finalized in 367 CE by Eusebius and given the stamp of approval by the Easter letter of Bishop Athansius of that same year. It was affirmed as "canon" in the year 382, making it the "New Testament" at the Canon of Trent at affirmed in the Syndod of Hippo. This codex, however, predates the Eusebius canon by 45 to 50 years if not more.
It is also not the standard reading that was being circulated at that time, nor was Revelation a popular book. By the time the Codex found its way to the Monastery of Saint Catherine, because of the fact that the readings were considered non-canonical and the fact that the collection contained non-canonical books not accepted by Christianity, someone that did not realize what the collection could mean to history tossed it aside to be burned with the rubbish.
Early writings do not mean "canonical," at least in Christianity. It might confuse some, and they do offer considerable insight, don't give me wrong. But to a Christian world where religious teaching did not stem from religious texts, and since at the time the Marcionist threat was all about "basing doctrine on Scripture," one has to develop a completely different viewpoint here.
The Codex Sinaiticus is different everywhere, not just in one spot. If a difference is what you are going to call out Christianity on the simple basis of one reading from this Codex, then it all falls because Christianity existed before this Codex.