I am a little confused.
With the dates that you provided, I found the 1900 census for William Taylor McCoy's children (he appears to have passed away):
1900 United States Federal Census > Texas > Llano > Justice Precinct 6 > District 128 > Page 184A
Dwelling 158/Family 159
Wm A Clark, head, white, male, b Jun 1872, 27, married for 6 years, b Texas, parents b TX, farmer
Mary A Clark, wife,white, b Dec 1869, 30, married for 6 years, bore 8 children, 5 surviving, b TX, father b Missouri, mother b LA
Sarah J, daughter, b March 1897, 3, b TX
John A, son, b Jun 1898, 1, b TX
George L, son, b Dec 1899, 5 months, b TX
Wm M McCoy, stepson, b Dec 1891, 8, b TX, father b TX
Ethel L, stepdaughter, b Feb 1893, 7, b TX, father b TX
So Wm Taylor McCoy's wife was Mary Ann (as you listed).Mary Ann is listed as the mother of Wm M, and was likely the mother of Charlie, and Calvin (since she lost 3 children), why was WT McCoy's wife listed as Susan on the tombstones?Were there be 2 Susans?Susan is not usually a nickname for a Mary Ann.
My guess is that Wm T McCoy died between 1892 and 1894 (when Ethel was conceived and when Mary Ann remarried).
If you haven't already, you might check with the Llano county courthouse to see if they would have any information about his death, or about the death of Susan Davis. If they owned land, there may be a probate record, which could list their wills or a list of their heirs.That could help further prove that Susan Davis McCoy was William T. McCoy's mother.
I don't know if Llano county had a newspaper at that time, but if so, there might be a notice of their deaths.
If William is really supposed to be part of that family, then there could be a few reasons for him not being with his family on the census:
a) He could have been living or staying with a family member or friend for a little while (esp if the family had a new baby), and didn't get enumerated with his parents
b) The enumerator could have left him off initially, maybe because he wasn't paying attention
c) The enumerator could have left him off when transcribing the information.I believe for most of the older censuses, the enumerator had to copy all the census information 2 different times, so that copies could be filed with the state, federal govt, and county courthouse.Over time, usually all but one of the copies was destroyed.So it's possible that he could have been left off that way
d) Could William Taylor McCoy have been adopted after the census date?
Good luck with your search.