Quite a challenge. The 1900 census shows that she was born in Texas in 1775 and her parents were born in Ireland. In the 1880 census there is only one Nellie who fits this description, Nellie Lynch in Galveston, dau of Edward and Mary.
I don't know where you have looked. To go through that many husbands, it would seem that there would be a divorce record (yes, people did get divorced back then) or a death or probate record for one or more of the husbands. I don't know if the Bellville, Waco or Tyler libraries have microfilms of the local newspapers, but these could be checked for obits. Check the deed records.
Cornelius (1900 census spelling) owned his home in PA according to the 900 census. So he should be on the deed records there. Sometimes the grantee deed will read "to Cornelius D. Smith of Austin Co, TX" (or wherever he lived.
But your best bet for identifiying Nellie's parents is to track down her death certificate. I got 76 hits for Nellie Smiths who died in Texas in my Texas VR database.