I see that Ancestry.com lists a marriage registration for Alexander and Elizabeth in Indiana. Have you seen it? Just wondering if it listed names of parents and/or birthplaces.
This is a tough one. How many children did Alexander and Elizabeth have? She must have been a widow, is that correct? She has children on 1850 and 1860 census with last name Winningham. There is a son Donald McPherson born about 1856? If Donald was Alexander's firstborn son, I lean towards Alexander's father being a Donald McPherson.