First, thought... If you dislike the word "obdurate" then do not read this book. Being a one thousand page book, you will have read this word between fifty and two hundred times by the time it's over. (Can someone get me a count on this?) Your brain will be bruised if you try to resist it. This word is out to get you.
As you already know, the novel is about a teacher and writer who goes back in time to change the past. But, as Stephen King reminds us over and over, "The past is obdurate and protects itself against change." In other words, King has personified "the past" so that it actually feels like it has a mind, volition and power to interfere with you if you mess with it. If you accept the premise of a character who goes back in time, you will have to give King credit for making the obdurate past one of the characters in his story.
Preventing the JFK assassination is our hero's quest. Jake Epping, who assumes the identity of George Amberson of 1958, actually has a two-step mission. Before interfering with Lee Harvey Oswald's assassination (his second objective), he must determine whether Oswald acted alone, or was actually indeed "the patsy" in a bigger plot.
We all know that King made his name by creating frightening nightmare tales and scenarios. Therefore, it would be out of character for King to make a straight story about a guy who tries to stop Lee Harvey Oswald. By necessity a King story almost requires a sinister element like the living past who is determined to thwart the hero's objectives.
There is a technique that writers use to help strengthen the bond between readers and characters. That technique is pain. That is, when a character, especially your hero, goes through pain the reader develops an increased empathy. Assuming we care at all about the characters, we cringe and we want to comfort them when they get hurt. So it is that King uses this tool to keep us in the game later in the book, for both Jake/George and his heart throb Sadie experience suffering in this story. (We writers can be so mean!) And though the suffering may just be a natural outcome of the characters' choices earlier in time, we readers know it is the inhuman past trying again to thwart our heroes from accomplishing their aims.
Right up to the end you'll be guessing what happens next. That's just how it is...
Photo captions: Images from the Cleveland Plain Dealer, 1963. Bottom right, the shooting of Lee Harvey Oswald by Jack Ruby. Click images to enlarge.