Most of the story comes from various sources of the legend. Even Mordred as the rightful king is supported by some sources, such as Boece's Historia Gentis Scotorum. That's why, at first, Lancelot joining the Saxons seemed a bit out of place, but then I realized: Guinevere's lover, tries to usurp the throne and allies himself with the Saxons, he is basically Geoffrey of Monmouth's version of Mordred
Also, both characters try to strengthen their claims to the throne by marrying women they shouldn't be allowed to marry: Geoffrey's Mordred marries Geinevere despite the fact that she is already married to Arthur, while Cornwell's Lancelot marries Norwenna despite the fact that she is dead.
As the story goes by, the portrayals of magic go from being simple tricks, often explained as such, to becoming more and more contrived coincidences and then outright inexplicable. Meanwhile, Unreliable Narrator Derfel starts going back to his old pagan ways in the segments that show the present time. So, his portrayal of magic becomes more fantastic because he himself slowly goes back to believing in it.
Why didn't the character of Derfel appear in medieval Arthurian myths? Because he's Lancelot - all his deeds have been attributed to Lancelot by later storytellers. The Lancelot from the medieval romances reads like a Composite Character of Cornwell's Lancelot and Derfel, having both Derfel's association with a sorceress named Nimue and his status as Arthur's champion, and Lancelot's homeland of Benoic and his betrayal of Arthur. Gwenevere and Ceinwyn seem to have been given the same treatment, becoming a Composite Character good queen who tragically falls in love with the king's champion.
Derfel actually does appear in a number of early Arthur stories as a knight who becomes a monk, but his role is minor, and he's dropped from all the modern adaptations.
In addition, Derfel's role in the Quest for the Cauldron strongly resembles a composite of the roles of Lancelot and Percival ... now tell me, what name does Percival sound like?
This does fit well with the recurring theme of songs and tales taking the place of the truth, that appears all throughout the books.
In the first book, when Derfel and Galahad are sent on a mission to see King Gorfyddyd, the King, as an insult, directs them to sleep in the women's hall, rather than in the men's, the implication being that they aren't warrior enough for the honour. This Troper isn't the only one who thinks bunking two muscular young enemy warriors in among the ladies (including the king's own stunningly beautiful daughter Ceinwyn) is a potentially disastrous idea, right?
Well, no-one said Gorfydyd was clever...
Plus, while he may not like them, he must know that Derfel and Galahad have solid reputations for honour.
