I've researched this William A. McCain and have contacted a more experienced genealogist to help me.
This started when I found a William McCane served in the 5th Mississippi Cavalry, Company I, which is the same regiment as my ancestor. This cavalryman was either captured or fell into Union hands after he became sick. He died in Memphis prison in April 1864. This cavalry man enlisted at Kosciusko, MS.
The problem is there were three(3) William McCain's living in Mississippi. Which one was Senator John McCain's ancestor and which one was the 5th Mississippi cavalryman?
When Senator McCain traveled to Univ. of Mississippi---Ole Miss---for his first Presidential debate, a newsman asked him if his ancestors owned slaves. His reply was that he didn't think so. Well, the McCain's owned a plantation called
Teoc, located East of Greenwood, Mississippi. In the 1860 Census, William McCain occupation is listed as a Physician. The slave census shows he owned 52 slaves. With this much wealth and some degree of medical knowledge, it would be very unususal for
this Wm McCain to enlist as a private. He did die in the middle of the war. Some theories by genealogist thinks he died of disease. He left a widow and a 12-year old son who carried on the family name. His widow was able to keep the land. Even though Senator McCain did not seem to know the history of his family's Teoc Plantation---even though he resided in Mississippi during WW2 while his Admiral Father and Admiral Grandfather was away---the descendants of their slaves still hold reunions which is attended by other members of the McCain family.
And who was the Wm McCane of the 5th Mississippi Cavalry? He appears to have been the individual who resided near Kosciusko prior to the War which explains why he enlisted there with other members of Company I.
There is a forum that discussed this.
Here is my webpage with a brief explanation.
http://custermen.com/DixieBoys/Miss5Cav.htm#SenatorMcCain