I have read your posting with interest.I don't have the same information that you have but it probably could go together at some point.
First, the name Sessions comes from Soissons, France.They could have been "Gaulish" but I rather suspect that they were more Norman. They could, quite possibly,have been "Norse Men," which is where the name Norman comes from.The Gauls were in that region at one time but so were the Norse Men. As you know, both were fierce. The standard guide to English surnames says that the name means from Soissons which is in Aisne.
The first mention of the name Sessions was in 1181 and that was on the Pipes Roll.The name appears again in 1190 in Cambridgeshire as did the first mention.The name is Esueillardus de Soissons, which is a pretty good indicator that the name comes from Soissons and not Sissions,which means son of Ciss.
The second mention of the name Sessions is Riulfus de Sessuns. The year is 1212 and it can be found in the Liber Feodorum, another census method.This time the name is in Berkshire, which is where the name seems to have stayed.
The dates would indicate that the Sessions name came about the same time as William the Conqueror--the best known Norman of all time.The Battle of Hastings did change the countrysie a bit.William wanted a roll taken, which was a census, to be aware of who was there, what they owned,where they lived, etc.
William died but the practice continued.The earlier rolls do not have the name--perhaps because they were not natives but came with him.By 1181, they had become English men and women.
I found this in a book about English surnames and I had an uncle who did some research many years ago.I have the information but I do not know where he found it.