To discern the fact of the matter about King's Arthur's court, facts must be drawn from the fiction of "The Court". With all of the knowledge we are learning about King Arthur, it must be discerned who some of these characters really are. With this being what one must do; one must see what history we have of the time. There is one history that must be consulted to find out what the truth of Great Britain's "great" King must be. The most historically believable character in Geoffrey Of Monmouth's story is Merlin. He is by far the most historically implanted character in all of the Arthurian stories. The historical implant my stem from the Celt Myrdynn, the prophet. This may affect the actual story, but most likely not. Therefore, the conclusion must be drawn the Merlin's real carnation did not affect Geoffrey of Monmouth's Story of Arthur.
The tendencies over time of people is to base myths, fiction on reality. Noah's ark probably was the Straight of Gibraltar breaking. Mount Olympus probably had fire at the top because of lightning, and Prometheus brought it down. Over time there were legends of some "Merlin" and storywriters probably made it their own.
He could have been an oracle in Scotland. He probably was called "Myrdynn" and this could be the missing link in this story. This story needs fact, or else it probably would not have written without him. Geoffrey of Monmouth wrote the Arthur stories.
The Scottish Merlin probably was 500 AD. Geoffrey was around 1200AD. With 700 years between them it is impossible to think that there could be a link. For it is this reason that Myrdynn could affect other stories, but not his defining story. 700 years is your great, great, great, great, great, great,