Shucks, cousins marrying was pretty common back - that is how I ended up the
descendant of three different 1700s North Carolina Helms brothers.
By the way, you missed a few Daniel folk on the 1860 census - there is a
James W. Daniel and a James Daniel, both with families - neither of them
have a son named William.
My reason for asking about Russell was that on page 77C of the 1880 Madison
County, GA census there is a Russell Daniel, born ca 1856 in GA, living with
the Wiles and Martha Kellum family. He is listed as a brother-in-law.
And then there is this:
William Kellum married Martha Daniel on 02 Jan 1874 in Madison County,
Lettie Carrithers (Malitia is shown as Litia in some census records) is
listed with her family on page 72C.
The James Daniel mentioned above is found on page 130 of the 1860 census,
and he has a daughter named Martha and a son named Russell. Of course, it
may be coincidence and not be your William at all.
The only William Daniel who really fits the age bracket for your William R.
is the son of Verona Daniel (who you have as Eurina). Her William is listed
as William J. in 1860, but only as William in 1870.
Another Daniel mystery <G>.