You can affect the future - cutting causes future scars. Except when you can't affect the future - Joe killing himself should mean Cid and Emily never met him and the other kids never died.
The way I saw it, you can affect the future, but not the past. Doing something to the past self affects things the future self does, but no matter what happens to a future self it can't affect the past self. So JGL right now killing himself so Bruce Willis doesn't exist has no effect on anyone from 1 instant before he did that. So everything Bruce Willis did still happened because it was in the past, not the future.