William Robertson had either a brother, or a cousin, I'm not sure which, named Samuel. Samuel Robertson married Mary Jane Semple in 1837, and they had a son, Samuel S., born 1 December 1840. John and Martha(Price)Smith, William and Sarah(Smith)Robertson, Samuel and Mary(Semple)Robertson, and Richard Elson's daughter Mary, who married Reuben Redman all moved to Scott twp., Mahaska County, Iowa.They didn't all move at the same time, but over a period of about twenty years. If you think this Samuel Robertson is who you are looking for, you'll want to go to the Mahaska County, Iowa, Genweb site, as there is a biography of Samuel in "Portrait and Biographical Album of Mahaska Co., Iowa, 1887".
I have learned from reading histories of Stark County that Richard Elson and John Smith bought the land and laid out the plots for the village of Magnolia. John Whitacre did the surveying. They were also partners in a flour mill, until about 1840, when John Smith moved to Iowa. You probably know this already, but the mill, built in 1834, has been in the Elson family for five generations, and was recently added to the National Register of Historic Places.
Sarah Smith was born in Brooke County, Virginia, in 1803. I have read that is where Richard Elson was born, so feel that these two families might have known each other before moving to Ohio. Let me know what you think.