I don't think it had top do with diligence, or lack of it. Christianity had an agenda, which started when it was obvious that the Church will not make it in Israel. The more "Christianized" scriptures enjoyed favor of the Church.
But we're talking about the KJV not the Geneva Bible, and the KJV used Beza's 1598 Greek Text for its translation
My understanding is that KJV is based mostly on Textus Receptus. The Geneva Bible is based on Beza's Greek.
In either case, the problem with the so-called Majority Text (Byzantine-type) is that it is 'too polished,' redacted, etc. compared to the Alexandrain-type.
My understanding is that the Greek Orthodox Church uses Byzantine-type for the Gospels and Alexandrian-type for subsequent books, which corresponds more or less to Codex Alexandrinus.
I don't think there was ever an official 'switch' after C. Alexandrinus came out in the 5th century. C. Alexandrinus was never a pure Byzantine-type canon.
The only reason the Byzantine-type was instrumental in make its way into the Protestant Bible is its numerical superiority. As you probably know, the Byzantine-type text represents over 90% of extant Greek biblical text.
However you are overstating your case
How many errors are acceptable in the "word of God?" Six, ten, one hundred...? Erasmus did what he did because he wanted to beat a deadline. He cut corners, even admitted so, just to meet the deadline.
Later on he accepted a forgery (Comma) as 'genuine' Greek text. One must seriously question his selectivity and ethics of 'means justified the end' approach. It is certainly unfitting for a Bible.
The first edition of KJV came out with admitted cornucopia of errors, along with the authors' admission that their work was not inspired. Why would such a Bible, based on Textus Receptus, a highly corrupted collection of already doctored Greek sources, become a 'standard" is beyond me.
As an Orthodox Christian I disagree with the Greek Church's continued adherence to Byzantine-type sources and to the KJV as its English 'equivalent.' But to do otherwise would mean the Church was wrong. It will never happen. Eventually, it may silently 'morph' into more neutral, Alexandrian-type sources, but never overtly or publicly.
My understanding is that this was a judgment call. It had been in the Old Latin texts, and so the question was: How did it get there so early and where would it have come from?
Since the Old Latin had been originally translated from the Greek vulgate beginning around 150 AD, it was thought to have been in the Greek vulgate at that time. Then about 70 years later when the Sabellian heresy was plaguing the East around 220 AD, it was believed to have been removed from the text to quell the heresy, since this verse was often quoted by Sabellians as their proof text. But since the Sabellian heresy was not as pronounced in the West, it survived in the Old Latin bibles.
It is cited by Tertullian, Cyprian, Augustine, and Jerome. Though Jerome did not put it in his Latin Vulgate, but it was put into the Vulgate around 800 AD from the Old Latin. It was included in Erasmus' 2nd edition and subsequent bibles including the KJV because of its long useage in the Latin-speaking church --- not deception or fraud or forgery, but tradition and a judgment call.
The conflation theory is dead.
The Majority text goes back to the 2nd century and is considered a legimate texual line, not an invented one.
So where is your proof?