The warning 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 Rolands and an end unto itself. What he found inside doesn't matter because he had nothing left to accomplish.
Nineteen is an arc number of the series. When Roland and Stephen King meet, it's on page 289, one of many numbers in the series that adds up to 19.
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.