My original reply was returned to me as undeliverable so I am trying again...
It is such a joy to hear from others who are interested in tracing the family history.To be honest I have just started in the last few months to refresh myself with the history starting with a book by Katy Roddie DeSoto and Laura Dingle Ewing called "Our DeSoto Family".It sounds like you have the same source or something very close.This book also describes all of what you have said about Manuel.One thing that does not seem to be valid is the reference to Santiago de Gritierce.I can find no records of such a place. On the other hand San Juan Dorron is indeed a parish of the current Santiago de Compostela bishopric.
For reference I am descended through Manuel from the following DeSoto's: Joseph Antoine Marcel - Joseph Simon - John Valcour - Alcide Valcour - George Elliot - Marion John.I see that our common ancestor is Joseph Simon DeSoto who was the father also of Neville A. who was the father of Willie Neville.
I would be happy to share anything I have beyond what you might already have on the family.I am focusing now on Manuel and the tie to Spain.Katy is going to try to dig up the Winston De Ville reference in Louisiana (La Voix des Prairies) which is the most direct reference in her book to the origins of Manuel.I would love to have any information which documents his exact name and origin.I am also working with the East Texas Research Center at Stephen F Austin University to do a search of their records of the Bexar Archives and other records for any of the DeSoto family, especially Manuel.Finally I have made contact with the keepers of the archives at Pontevedra, Spain (the town near Manual was supposedly from) and Santiago de Compostela (the bishopric).I have also faxed a letter the the Archivos General de Indas in Seville, Spain, which supposedly has a list of every Spanish passenger to the Americas from 1500-1800 and often their parents, place of birth, intended destination, and intended occupation.I will leave Belgium for Santiago de Compostela and Pontevedra Spain on Friday so if you have any suggestions or further information before that time I would love to hear from you. I will be sure to pass along anything I learn in Spain.