I have no idea what county in Kentucky was his birthplace. I tried narrowing down the heads of household with sons in his age range in the 1830 and 1840 census, but there are more than one household that fit that slot. He also could have been in either Kentucky or Missouri during that time.
My inference about the mother being Elizabeth came from the census you quoted, yes -- Pulaski Co., Mo. 1850. The Rays and Watsons intermarried and there are, as far as I can determine, two Harriet Rays and one Harriet Ray Watson, that I know of.
I don't think Cornelius and MaryAnn were divorced - her name is difficult to make out on the census materials (it could be Margorie), but anything is possible. I have certified copies of the marriage records from Miller County of him and Elvira Johnston.
Henry Franklin Watson and Rosa Jane Bodine are my ggrandparents (both b. Mo.); they were in the Okla. land run in 1889. They had migrated into north Texas prior to 1899, which is where my grandfather, Louis Richard Watson, was born (Sunset Tx - 1885).
It is possible that the MaryAnn Watson listed in the Blackwell will could be Cornelius' wife -- I have no idea if she died or if they were divorced (isn't that rare for that time period?). I am inclined to say it is a different Mary Ann Watson, though.
I have all the records that Camp Morton could supply, including a death record; plus I got copies of all the documents the cemetery had - short of going there and physically going through whatever papers they have in the storage archives. None of them list any personal information, however.
I am not sure Elvira got a widow's pension (I hadn't looked into that) - she had remarried by 1870, and had more children with Joel Wilson, and then disappears from the census records. I haven't researched any Wilsons.
How could I get any information on the widow's pension records, if Elvira got a pension? I would bet that she did, because of the young ages of her children.
Cornelius was in the Confederate Army, served in Wood's Cavalry, late in the Civil War. I have not been able to find out much about Wood's Cavalry. I have documents showing where he was captured, but that's about it.
A young woman named "Avarilla Baker" is listed on the census with the Cornelius household, and I have no idea what the relationship was with her. I wondered if she and MaryAnn were sisters; the ages would fit well. That would then imply that MaryAnn was a Baker.