Stephen King as Gan..

I just finished the part of SOS where Eddie and Roland pay visit to Sai King, and put him under hypnosis.

And I am left with a few questions and some amount of confusion...

This is what I took from this part of the book:

Gan is "The White"... the forces of good.... ka...

Somehow Sai King, beyond his own comprehension or even awareness, is the vessel through which Gan operates. What Sai King believes is only literature and fiction, is actually him driving Roland and his ka-tet along the beam to the Dark Tower...

But, I am kinda confused about this... it seems as though Roland is being driven by King's novels. But had Sai King never been born (hence, never writing the Gunslinger or it's sequels), would there not be a Roland? In other words, I am gathering that Roland would have existed whether or not Stephen King had ever written the Gunslinger series... but perhaps without Sai King, Roland would never have been able to reach the Tower?

Also, what is and what isnt a product of King's imagination? I mean, did Hey Jude exist in Mid World already, or does it only exist there because King created a piano player character named Sheb who was playing it at a roadhouse in Tull?

Did King also drive Walter/Flagg? We know he drove Roland, but how much of what Flagg did... or the Wolves, was controlled/driven by King as well? To me it seems as though the Wolves resembling Dr. Doom from Spiderman comics is something King thought up.... but would the Wolves still have looked the same without King writing "Wolves of the Calla"? The Wolves arent part of the White.... but it still seems like King "came up" with them.

Ive probably overthought this, and should just get back to enjoying the series, but Im kinda perplexed here a bit, and hopefully my questions or clear and not confusing.