The Snicket Warning Label before the Coda describing what Roland finds in the Dark Tower makes more sense when you consider that Roland has already saved the Tower by destroying Algul Siento. Getting inside the Dark Tower was a personal obsession of Roland's and an end unto itself. What he found inside doesn't matter because he had nothing left to accomplish.
It urges the reader to let the story end with Roland walking into the Dark Tower. It adds that the following pages are only for those readers unwilling to let fictional characters go to the Grey Havens (a Lord of the Rings reference) and take their rest. So in other words, when Roland reaches the top and is reset to the beginning of the first book, it can be seen as a You Bastard take on readers who can't let a story or characters go and start reading again from the start.
Roland is, in some ways, an Author Avatar of Stephen King. When they meet, they're described as looking very similar, not quite twins but easily mistaken for father and son. Roland's father's name is Steven. Also, a characte explicitely notes that the similarity in names of the Crimson King and Stephen King.
The idea that Roland's constant recourse to ka throughout the previous books is not out of any genuine sense of destiny, but simply because he has trodden this same path so many times that on some level he remembers what's coming.
In-universe example in The Dark Tower series - when Eddie and (O)Detta come to Roland's world, the first explanation they get for its state of repair is "The world has moved on". Both new arrivals regard the wide open spaces and total solitude of the place very romantic. Then they get to Lud, and my, their opinion changes...