What? He couldn't see what was going on earth since in Heaven and had to come down to see?
I agree that all hermeneutics is adding to scripture with the intention of aiding understanding. I do not think the admonishment about adding to scripture ever applied to any book beyond Revelation. Indeed, we need to place ourselves into the shoes of every person mentioned in the Bible - hopefully, someday it will become our own autobiography, so to speak.
Quite right that refers to that book only.
While you are right about that, you've added to what I wrote, assuming I meant Revelation when I did not.
But now look at passages from the beginning and end of Revelation.
Revelation 1:3 Blessed is he that readeth, and they that hear the words of this prophecy, and keep those things which are written therein: for the time is at hand.
Three steps are needed to be blessed: Reading, hearing, and keeping the things. Reading the written words is the first step. Is the second step hearing those humans words spoken aloud, or is it hearing what the Spirit says? It's the second.
Revelation 2:7 He that hath an ear, let him hear what the Spirit saith unto the churches; To him that overcometh will I give to eat of the tree of life, which is in the midst of the paradise of God.
Revelation 2:11 He that hath an ear, let him hear what the Spirit saith unto the churches; He that overcometh shall not be hurt of the second death.
Revelation 2:17 He that hath an ear, let him hear what the Spirit saith unto the churches; To him that overcometh will I give to eat of the hidden manna, and will give him a white stone, and in the stone a new name written, which no man knoweth saving he that receiveth it.
Revelation 2:29 He that hath an ear, let him hear what the Spirit saith unto the churches.
Revelation 3:6 He that hath an ear, let him hear what the Spirit saith unto the churches.
Revelation 3:13 He that hath an ear, let him hear what the Spirit saith unto the churches.
Revelation 3:22 He that hath an ear, let him hear what the Spirit saith unto the churches.
Revelation 13:9 If any man have an ear, let him hear.
We could read the book a thousand times without benefit if we don't hear what the Spirit says. Others can offer suggestions, some may be right and some wrong; but it is not possible for anyone to do another's hearing for him. If I say something right as a hint, I expect the Holy Spirit would whisper, "Yes, that's right." If I'm wrong, the Spirit may say, "That's not right" or nothing at all.
Then when the truth is revealed, the person with the spiritual ear is expected to do things. When John writes "things," that's what he means. Now with that in mind, read the part at the end:
Revelation 22:18 For I testify unto every man that heareth the words of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book:
19 And if any man shall take away from the words of the book of this prophecy, God shall take away his part out of the book of life, and out of the holy city, and from the things which are written in this book.
The person who "hears" what the Spirit says is going to be held accountable.
Do not add "things" to the list of things to do. Do not take away words so you can ignore them after hearing and understanding them.
I try not to add words to what other people say. I want to understand them. If someone says, "I saw Bill at Walmart's this morning," I don't add "and my wife" to the statement so it says, "I saw Bill and his wife at Walmart's this morning." Neither do I take away words to have him saying, "I saw Bill this morning." If I care about the person, I want to understand him so I have to pay attention. If I pay attention to what people say, why wouldn't I pay more attention to the actual words in the written Bible, thinking they mean something without my adding or subtracting?
The idea of neither adding nor taking away is found elsewhere.
Deuteronomy 4:2 Ye shall not add unto the word which I command you, neither shall ye diminish ought from it, that ye may keep the commandments of the Lord your God which I command you.
First the literal word, then understanding, and finally obeying for again:
James 1:22 But be ye doers of the word, and not hearers only, deceiving your own selves.
In Jeremiah's day, people had the written words of Moses, word for word; but there were too many false commentaries, adding and subtracting things so that when people read the written word, they couldn't Hear the Spirit. Their minds were too cluttered by the lies they read from the pens of men. They did not have the Living Word.
Jeremiah 8:8 How do ye say, We are wise, and the law of the Lord is with us? Lo, certainly in vain made he it; the pen of the scribes is in vain.
9 The wise men are ashamed, they are dismayed and taken: lo, they have rejected the word of the Lord; and what wisdom is in them?
David said the Law (Torah) was perfect. There is no need then to add or subtract words.
All that being said, we might wonder why the Book of Revelation in the oldest manuscripts we have are so varied. Some books in the New Testament have only a few variations. Revelation has so many, it may have more than the rest of the New Testament put together. Some things somehow got added and taken away. Still the manuscripts agree with each other enough, we can derive a fairly good idea what what the original was; and if we depend on the Spirit, if we wait to "hear" with our spiritual ears, the words we do have are enough.