How nice to hear from you! Maybe we can work together to solve some of the Koyl puzzle. Here are some things that I found in my paper files:
I don't have anything that says specifically that John was the son of Nelson and Mary but based on what you do know, I would say you are right because John was a brother to Rodolphus and we know that Rodolphus was a son of Nelson and Mary. I found Nelson Koyl in the 1871 Canadian census. He was living in District: Elgin East, Sub District: Dorchester South and was 60 years old. He is on microfilm # C-9899 on page 29. According to Myron Koyle who has been putting all of this together for longer than I have: Nelson Koyl was born February 11, 1811. Myron listed one son: Alexander Koyl(e), b. 1836. Since then I have found some other children of his: Hannah M., b. abt. 1833(she married Benjamin Willis in 1861); Alice Koyl(age 17 at the time of her marriage in 1865 to Wm. Atkinson), Rodolphus C., b. 1848 and your John, b. 1842. There are some other children that need sorting out: Mrs. Marth SMITH, Mrs. Florence TRAVIS, Mrs. William NICHOL, Mrs. J.A. ROWE and Mrs. Thomas DENNIS. I'm not sure how all of those tie together but the first 3 are sisters of John and the last two are sisters of Rodolphus who were apparently brothers.
Zina Koyl does not ring any bells. I can tell you a little more about Nelson Koyl's ancestry when you are ready. In the mean time, I will look on some of my Canada CD's to see if your John comes up there.