I agree that King's book is a work of fiction and not intended as a primer on the JFK assassination. King has, however, both in his book and at public events layed out the case against Oswald. That evidence is persuasive. There is no real conjecture about Oswald's guilt from King's perspective either in the context of this book or outside it. It is not an "alternative reality." It wasn't ambiguity regarding Oswald's guilt that made him select the JFK assassination to drive the fictional plot but rather what would happen if a major historical event was altered (i.e. Oswald was stopped from assassinating JFK). I agree though that the topic of Oswald's guilt is generally pointless to debate. My experience with JFK conspiracy theorists is that no amount of evidence or common sense can ever change their opinion. It is a matter of faith-based belief in how the world works that this had to be the work of sinister forces that control all major events. If they were capable of exercising reason and common sense, they would not be conspiracy theorists to begin with. So the better argument for not engaging in these endless debates in my opinion is that no one will ever change their mind. The history books record that Lee Harvey Oswald assassinated JFK. Every official investigation has reached that same conclusion. The evidence overwhelmingly supports that conclusion for any reasonable person who takes the time to study the basic case. The fact that some can never be convinced of this obvious conclusion is meaningless when the facts and circumstances support Oswald's guilt.
There's always going to be people that don't believe something unless it is 100% confirmed and there's absolutely no way to deny the truth. While the history books do say Oswald assassinated JFK, there's no doubt that there are some serious odd things that happened that can lead to question, which can lead to doubt... people have the right to be curious and not just accept something because it seems "the most likely thing to be true." Thats just my take. I believe that there are enough things that happened before, during, and right after the assassination that there is still a sliver of doubt about what truly happened on that infamous day.