You are correct.
Abraham Lincoln, William McKinley and John F. Kennedy all are tied for second place.
I dunno. The 'Toon is probably a close second, depending on how widely you define "assassination attempt." Remember that freak that stole a Cessna and plowed it onto the White House lawn in September 1994? Another nut opened fire on the White House (from outside the fence, IIRC) just a month later.