Brad, I dont have access to the 1921 census, but Sc00p says that it would appear that the name Mabel Harmer was written and then changed to Mary Elizabeth Harmer. Others may be able to comment on what they see.
This adds weight to my theory that Mary Elizabeth Harmer, nee Sayles was the same person as Mabel Ethel Brooks, who is the mother of Joseph Brooks born 1915. (The name Mabel can be a diminutive for Maria.)
Theoretically, the father of the child would be Joseph Brooks born 1875.
All of that appears to be plausible at the moment, but the only way to confirm it for certain, is to find the DNA matches. If your grandmother is open to taking a DNA test, you might make additional progress.