Death certificates are only as accurate as the informant's knowledge of the person. In other words I'm suggesting William's father's name may not have been Isaac. Since his father was out of the picture when William was young his knowledge of him may not have been accurate.
I say this because the 1860 census of Giles County, TN lists a Frederick Kussman 25, born in Bavaria. He may or may not have been related.
Delia and family are listed with her father in the 1860 census under the name Kusman. This suggests she was either separated from her husband or he was deceased. If he was deceased then obviously someone else was the father of Fritz/Fitz. Fritz is sometimes a common nickname for Frederick.
Helen A. Cushman married John F. Skeggs on July 5, 1877 in Giles County (see familysearch.org). She died in Nashville on Jan 24, 1883.
Marriages from Giles County 1850-1858 do not appear to be online yet. If the county did record marriages back then they might be at the county courthouse. Delia's marriage record would help to confirm her husband's first name. Also there might be other records concerning what happened to him at the courthouse such as a death record or divorce records if that happened.