Maybe we need to find you some!
So the answer really is not so simple of its not being found in earlier TR revisions, since you do not hold them as any kind of standard.
With that being an unknown on your part, it might be better
not to claim, "The AV makers simply followed the TR without verifying it."
Without researching this further, one might just think Beza was reading along through Revelation 16:5, didn't like what it says, and decided to change it. That is how what you wrote comes across to me. Further, you say it is "not found in any of the mss. used by Beza to make his revision." However, is that a correct assertion? I am not sure that it is. I have room to learn more about this, especially if Beza wrote about this beyond what is in the
Novum Testamentum of 1598. Also, if I could read Latin with better understanding I might feel stronger about this. Nevertheless, I have to muddle through as best I can. Here is the note in his 1598
Novum Testamentum.
[In addition to my Latin deficiency, some of it is just hard for my old eyes to read.] I will summarize what I think the note on verse 5 says. Most texts generally read καὶ ὁ ὅσιος, but he thinks that reading is corrupted. Part of his conjecture is that the triadic statement appears in the other four places (Rev. 1:4, 8; 4:8; 11:17) and he gives a reason why the reading would be ὁ εσομενος (shall be) in Revelation 16:5 instead of ὁ ἐρχόμενος (is to come) as in the other four places. However, there is a relevant statement --
Itaque ambigere non possum quin germana sit scriptura quam ex vetusto bonae fidei manuscripto codice restitui nempe ὁ εσομενος -- that appears to say that he has restored the reading as found in an ancient codex. (My rough translation of that is, "Therefore, I cannot doubt that the correct reading is rather as I have reinstated it, from a trustworthy old codex/manuscript, namely ὁ εσομενος.") To be fair and clear, others apparently do not read it this same way, possibly because we are not presently aware of any such codex.
[Note: Beza's 1598 Greek text is available online, but I have lost the link. It was on a page where you could download it, which I did, but have now lost the url. If I can find it, I will post it here. The work is presented in 3 columns, Greek on the left, Beza's Latin translation in the middle, and the Latin Vulgate on the right.]