If the past doesn't want to be changed, then why was it so easy to tell and convince the guy he was from the past and trying to prevent JFK's assassination?
l see where you're coming from, but think about all of the other things that are 'changed' in the past, even in the book.. Jake places bets, meets and falls in love with a woman of the time period.. makes a lot of moves and 'butterfly effects' so many seemingly small events. To me, telling a guy a wild story and making a friend seems within the same boundries.