You’ve raised some interesting questions.
A great deal of what we recognise as Arthurian derives from later medieval invention in the French-speaking and Anglo-Norman literary worlds, that is to say they don’t really appear in the early British/Welsh material even though they draw on that older lore. You could say that this Arthurian tradition starts with Geoffrey of Monmouth – a person who was described by William of Newburgh as an “impostor with a love of lying”! Poor old Geoffrey!
As for the Christian influence creeping in … I suppose we have to acknowledge that these tales were written down in medieval, Christian Wales and storytellers, as always, would have used the language and mannerisms of the times so that their audiences might relate to them. Although these stories draw on older folklore, mythology and legend, in many senses they are also very much about the world in which they were written, i.e. the Christian medieval courts of the Welsh kingdoms. In addition, material that has been handed down from generation to generation can also be eclectic – conserving very archaic material, later additions and contemporary issues all being reworked and retold to suit their audiences. If you think about it, modern cinema does this all the time. I suppose this now leads us to the question as to what a myth really is and what purpose it serves – reading beneath the surface.