Chris, I will give you the Stamdard Answer about the St Clair/Sinclair spelling/pronunciation that I have been given by others for the past 20 years or so.
"Rollo - the first "Sinclair" - took the name of St Clair, and it was used in the sense of "Rollo of St Clair" - the name of the village in France where he cut a deal with King Louis. The French pronounce the words Saint Clair, as "San Clair", but the English pronounced it something like "Sin Clair", and of course people in England prononunced all of the Catholic saints this way. In converting these sounds to written form, the French came up with "Saint Clair" or "St Clair", while the people across the channel ran the two together and we have "Sinclair", of course, and even that has been spelled 92 differant ways by people all over the globe. Actually, some of the earliest referances to Rollo and his descendants still used the medieval forms of address, such as "Richard Strong-Arm" and William Le Blond" to describe people in that line who were not also called St Clair until after they had been in Britain for some time. The earliest of our lines in Scotland (of record) built a castle in Midlothian and used the St Clair name for many generations there. Even as people named Sinclair
appeared, many of the line still in France and their descendants elsewhere still use the "St Clair", and we all know they mean the same.
I believe "De Sancto Claro" may be an appellation used in their context, but I'm not sure it refers to the Saint Clair, who was an English man who was made a Saint by the Normans.
If anyone comes up with more logical or factual explanations, I'm sure we would all like to add this to our trivia box.
Aye yours, Ray Lower, Folsom, CA