You may well be right in terms of proof. It's generally very hard to prove anything in cases like these. I do believe that Oswald was involved, but only that he was set up to be the patsy.
If someone else was involved, it would have leaked decades ago.
I believe it did. I can't speak for Roger Stone's book other than the article on it that I linked to previously, but I found Jim Marrs' book Crossfire, originally published in 1989, to be most informative in this regard.