The Profiles in Courage thing has always set off my BS detector. It's like a guy comes in off the street and gets named the starting quarterback for the Super Bowl and wins, having never played a game before and never playing again. The rumors I'd heard were that Theodore Sorenson, his primary speech writer, wrote most of it.
After reading your reply, I realized I have heard of Sorenson as the ghost writer. I decided to look it up and indeed he is often credited as the writer.
I noticed some think it was both of them collaborating, by both I mean Sorenson and Sclesinger, not Sorenson and Kennedy.