Hi - Thanks for the details. I agree that John son of John Corkill & Margaret bapt Maughold 15 Sep 1850 is your John James - the birthdate of 7 Sep 1850 is perfect, because he would still have been aged less than 7 months at the time of the census.
Perhaps he or his parents added the "James" to his forename sometime after his baptism because of the number of John Corkills around, including his own father.

I also agree that having his biography written in his lifetime is very likely to be accurate - how lucky to have this.

Sue