The early church read the book of Revelation in its entirety every time they met

But you said

There was for almost 900 years only one text read in all Christian churches.

I hope you see the difference.

There is a curse spoken at the end of Revelation to anyone who tampers with the contents of the Bible.

Have a look at "Misquoting Jesus" if you can find it. As I recall he says there are - or were at the time of publication - over 30,000 remnants from early versions of the new testament, and there is a lot of variation among them. He cites specific instances of passages being modified to suit the politics of the time and place.

The King James version itself is an example: "...a product commissioned to reinforce a clear-cut royal political agenda, to be done by elite scholarly committees, and reviewed by a self-serving bureaucracy, with ultimate approval reserved to an absolutist monarch." https://www.christianity.com/church/church-history/timeline/1601-1700/story-behind-king-james-bible-11630052.html