Success! At least some. This morning, I received a reply to my enquiry to Oakland Cemetery, after which I phoned them with further questions, although they had limited information.
First of all, this is what they have regarding Mary's burial in 1915. Looks like you were right, Sandra, and you did find the correct John Johnstone in 1938! Thank you.
8D Mabel Plot John Johnstone
Johnston, Jennie McKay buried Oct 25, 1922 (I don't know who she is, unless George's wife)
Johnstone, John buried Oct 29, 1938
Johnstone, Mary Barbour buried Aug 21, 1915
Johnston, George buried Dec 8, 1956 (John did have a brother George born Nov 13, 1870 in
Scotland)
In the Ashburton Plot, Row A, Block E, Grave#5
Irvine Johnston, date of death, Jan 20, 1916, date of burial Jan 22, 1916, age 37, born Scotland. John's brother Irvine was born Feb 7, 1876 in Scotland so would be age 39 but this MIGHT be him, although there were many Irvine Johnstons, some others also in Yonkers.
They also had a Janet Fish Johnston who died Feb 16, 1917, buried Feb 18, 1917, age 62, born Scotland, last residence Yonkers, lot owner Eliza F. J. Wilson.
I am confident that John C. Johnstone, did die in 1938 and that is him buried with Mary Barbour. As for George, maybe it is his brother who was still alive in Scotland in 1891, single, and was a gas fitter.
I am thinking it is possible Irvine who died 1916 could be John's brother but needs further investigation. If him, where was he in 1910? And where was his wife Jane?
Peggy