Thanks for explaining about the "H" in David "H" Lowe. I wonder what it stands for. Anyway, it should help identify him if his birth record is ever found.
Re. the article from the Green County, Wisconsin, history book -- it mentions William Lowe in Mount Pleasant, Green County, Wisconsin, in 1849. The census we posted for William is William Lowe in Mount Pleasant, Green County, Wisconsin in 1850.
Pursuing historical sources in Green County might yield more about him.
Very important to find your cousin, the family historian Marion (IF she has proof with primary sources that "your" William Lowe married Mary Sanderlin) OR to find the primary sources yourself that prove it.
There is a Sanderlin child in the 1860 census with William Lowe's wife Mary - this lends credibility to the idea that Mary is Mary Sanderlin (or a similar surname).
Mary Lowe may have died in Monroe County, Wisconsin; at least we last saw her in 1860. Her death record may tell her parents' names.
Also try to find records for her children on Family Search and Ancestry.com and on the Wisconsin Genealogy index. This includes their birth records, marriage records, death records, and obits. Somebody sometime must have mentioned her maiden name (we hope).