This ending is an inclusio to the beginning of the book:
Blessed is the one who reads aloud the words of this prophecy, and blessed are those who hear, and who keep what is written in it, for the time is near. (Revelation 1:3) [ESV]
I warn everyone who hears the words of the prophecy of this book: if anyone adds to them, God will add to him the plagues described in this book, and if anyone takes away from the words of the book of this prophecy, God will take away his share in the tree of life and in the holy city, which are described in this book. (Revelation 22:18-19)
The initial application would be on the original circulation of the letter which John was instructed to write to the seven churches in Asia; what he writes is to be read, heard, and kept. If anyone fails to follow the opening instructions, they will be "cursed."
Beginning (1:3) Ending (22:18-19)
Blessed "Cursed"
Reads aloud [Implied is the reader cannot add or take away]
Hears Hears
Keep what is written Lose what is written
[Implied is John wrote exactly] [Implied is a copy must be made exactly]
Therefore, the initial emphasis is not on future written copies, but reading and hearing. In other words, if John sent what he wrote with another who was to read it in the seven churches, the reader must be faithful to read exactly what was written without adding or taking away any words. Similarly the initial audience is to keep what they hear without making any change. That is to say, the prohibitions and consequences apply equally to the one who read and those who hear. Thus both reader and hearer are equally responsible for keeping what was written and failing to do so would have dire consequences.
The question of English translations is two steps removed from the text:
- Making a copy of the original document
- Translating of a copy (or the original)
As to making a copy, the implication of the ending is that John was faithful to write exactly what he was told. This is confirmed earlier:
And when the seven thunders had sounded, I was about to write, but I heard a voice from heaven saying, “Seal up what the seven thunders have said, and do not write it down.” (10:4)
John's ending is modeled after his experience. He was prepared to write what he heard, but was told not to write it down. So he kept what he heard. Therefore, the initial audience knows they are receiving exactly, and only, what John wrote. What is implied in John's original writing, obviously carries over to anyone making a copy. Given the original method of distribution, it would be not be unreasonable to assume each of the seven churches immediately copied the letter in order to have a written record on hand of what they are to do.
Obviously, the ending would also apply to anyone making a copy. In his commentary Gerald L. Stevens says:
A curse formula is invoked about the maintaining the integrity of the contents. In the ancient world, books were at the hands of copyists, and formulaic endings like this were common to try to forestall temptation to taper with the words. The Letter of Aristeas purports to relate the production of the Septuagint version of the Old Testament. The author records the following at the conclusion of the project.
"And when the whole company ex[pressed their approval, they bade them pronounce a curse in accordance with their custom upon any one who should make any alteration either by adding anything or changing in any way whatever any of the words which had been written or making any omission. This was a very wise precaution to ensure that the book might be preserved for all the future time unchanged." (Aristeas 311).1
However, translating a copy from the original Greek to any other language cannot be done with the same word-for-word precision. This is a result of the differences inherent in different languages which makes translating without adding or subtracting impossible.
Conclusion
The instruction applies directly to hearing and reading and, by implication, to making copies in the Greek language. It does not directly apply to translating the Greek into English, or any other language. However, the primary emphasis of Revelation is on "keeping" and doing what it says. So the instruction indirectly applies to all translations; to the extent a translation failed to correctly convey what a reader and hearer is supposed to do, the "curses" at the end apply.
With respect to the imagery in Revelation 12:15, the primary object is water which is not usually symbolic with the word. However, the church at Ephesus might make such a connection:
that he might sanctify her, having cleansed her by the washing of water with the word
(Ephesians 5:26)
If the water which comes from the serpent is symbolic of a counterfeit attempt to "wash" the followers of the beast (or serpent), then it is more likely it is a false gospel, not a poor translation of Revelation.
The important connection in Chapter 12 is with "keeping" the commandments:
Blessed is the one who reads aloud the words of this prophecy, and blessed are those who hear, and who keep (τηροῦντες) what is written in it, for the time is near. (1:3)
Then the dragon became furious with the woman and went off to make war on the rest of her offspring, on those who keep (τηρούντων) the commandments of God and hold to the testimony of Jesus... (12:17)
The emphasis is on keeping the commandments, doing what is written. This may require more or less words in English to properly convey. Moreover, unless there is a global conversion to the English language (a reversal of the Tower of Babel), there would not be one standardized "word of mouth" transmission for everyone to follow.
If the ending (and beginning) of Revelation is used to critique the plethora of works, it would be on the over emphasis given to those texts which have no direct practical value. So much of what is written about Revelation is on what will happen, to the detriment of communicating what one is supposed to do at all times, regardless of what happens.
1. Gerald L. Stevens, Revelation, The Past and Future of John's Apocalypse, Pickwick Publications, 2014, p. 557
