Hi Alex. I read the link you sent me to. It is very interesting. I doubt there were any butlers in the family, they were said to be well to do. The Elizabeth Sevier you are looking for can not be the ones born in USA. There is very little information about Don Juan De Xavier. He moved from France to England and changed the last name to Sevier to appear more English. I have not seen the name Tant anywhere in the Sevier Family History, a book written by someone who has passed on. I can search more on the France to England connection and see what I come up with.
Some more info that might help you. Don Juan De Xavier was b. Abt. 1652 in France. He married "Marie" b. Abt 1654. Child of the above: Valentine Xavier, b. 1678 Navarre France. Valentine Sevier was born in London, England in 1712, the oldest of several children of John Sevier (Jean Xavier) and Mary Smith. this John Sevier, a Huguenot, fled France just before, anglicized his name (pronounced like the adjective "severe") and settled in London. His son Valelntine was christened there in 1712, in St. Giles Cripplegate Church in London which still stand and is inuse today. It was severely damaged in the bombing of London during WWII but is now completely restored. It is locaated in the "Old City" area near famous St. paul's Catherdal. In 1740, Valentine and a brother William, "took ship" for the American Colonies. So it appears that there were several children still in London but no other names are listed. Maybe these children were christened there as well and there might be a record of this at the St. Giles Church. This information came from John Sevier "Jack" Gibson. I have no idea how to search for records in France and England. Good Luck!