When doing Textual Criticism while also believing in providential preservation, it is important to distinguish 3 things about the church catholic throughout history:
1) Textual Conclusions
2) Epistemology
3) Methodology
The first 2 are ever changing. But 3 has been quite stable. The church has been applying External and Internal Evidence as a methodology for negotiating textual variants since the 2nd century. It did not change at the Reformation. And it is ongoing today. Now the data and amount of info has increased greatly, but not the methodology.

I just read a @tbsbibles article applying Dr. Robinson's test-tube critique to the NA27, where the author mentions a similar problem of only a single MS support. Funny, as I read the ECM on 1 Jn, I note a few places their TR has only 1 Ms supporting several of its readings.
1/4 Only GA 2318 reads exactly as the TR at 1 Jn 5:7-8. And that Ms was written well after the KJV was published! There is a close second (2473), but is not perfectly identical, and it too is was written after the Elzevirs 1633 printed TR.
2/4

I find it very interesting that one of the most unstable and varied passages among the many TR editions is the Comma Johanneum. It doesn't appear precisely as in the Scriv TR until Stephanus's 3rd edition (1549)! Even then, 4 of 10 Beza later eds would not match it exactly. 1 Jn 5:7-8 is the most jumbled mess of a textual apparatus I've ever seen... and that is just from the TR editions!!!