I don't want to give to much away- but from the protagonists point of view he wants to be 100% sure it was Oswald alone, he starts off %95 sure it was Oswald, but spends years spying on him to be %100 sure (Otherwise he could have simply saved himself a lot of trouble just killing Oswald at the first chance he got- the time travel is to 1958 only- so he has to wait 5 years before the assassination)- the concern of the protagonist is that George de Mohrenschildt is part of the plot...

In the postscript King has little time for the conspiracy theorists- he says his considerable research leads him to believe Oswald acted alone...