Selkirk's diaries (copies) are housed at the archive section (Lambton Room if I remember) of Weldon Library - University of Western Ontario, London, Ontario.
I have interest in your finding out more, as Mary MacDonald m. Allan MacLean was I believe Donald "Tailor" MacDonald's sister. Mary's mom also came to Canada with Allan and Mary. She died shortly after arriving at Baldoon from illness (malaria? fever?). Her death was definitely recorded in the letters to Selkirk!
I am also related to the mystery family through my Turner family, and remember my great aunt telling me many stories regarding the mystery.
I have tried a number of times to connect your John Moor McDonald, but no success.