When considering ancient written sources, including scripture, the modern emphasis is to assume the null hypothesis that if they are 'wrong' (i.e. factually inconsistent with other sources) in one respect, then they are 'unreliable' hence should be ignored.
Well, this is one assumption; but it is likely that ancient writers - who were mostly religious men, mostly apparently deeply religious men, for whom writing was a semi-sacred activity - were more truthful than modern historians - who are essentially just clever careerists.
More truth-full - but truthful in that way which is natural and spontaneous to humans - and in a way that is different from the post-religious and pseudo-scientific idea of truth which has dominated the West for recent generations.
The truth they are attempting to convey is the essence of the story, not the specific details.
This is not an un-sophisticated way of writing - it is indeed perhaps more sophisticated than modern culture, in the sense that Chaucer is more sophisticated a writer than anyone alive; and everything which Chaucer wrote which can be checked against other sources shows many changes of detail - deletion, expansion, new material etc.
But it is likely that we modern reader lack the training and skill to read this kind of writing as it was read at the time - and we probably miss the truth of it: miss that which was most important to Chaucer's contemporaries.
(CS Lewis remarks that the interlace method of composition, so popular in the high Middle Ages and extending to Spenser's Fairie Queen, was popular for centuries - yet is generally too difficult for modern people to follow: we cannot hold the structure in our heads, nor recognise the logic of it. There are likely to be many other examples.)
When an ancient writer is being truthful in a 'literal' sense - or the nearest equivalent, then he simply copies, or he memorizes word for word (or as closely as humanly possible, under the circumstances) - this is how certain specific prophecies or ancient laws are transmitted. They may be cast into a song or chant, then repeated and repeated until learned by heart.
When transmitted in this fashion, there is often inconsistency within and between texts - because the focus is on the specifics and not the generalizability.
Think about proverbs or maxims. These are traditional ways of transmitting advice, rules for living - all are accepted as true (none are denied or dismissed); yet maxims often (apparently) contradict each other; and the wisdom is in knowing which one to apply in a particular circumstance.
This is demonstrated in the Lord of the Rings by Tolkien (who knew allabout these things) when the Fellowship is about to set out, and Elrond and Gimli engage in a 'battle of proverbs' (as Michael Drout calls it). Elrond says that no member of the company (except Frodo) is required to go all the way to Mount Doom. Gimli dissents, and replies with the proverb "Faithless is he who says farewell when the road darkens"; to which Elrond replies "Let him not vow to walk in the dark who has not seen the nightfall"; Gimli: "Yet strong word may strengthen quaking heart"; "Or break it" - finishes Elrond.
This shows that all traditional wisdom is regarded as true (nobody wise denies the validity of a proverb) 'despite' that proverbs superficially contradict one another.
This should also be assumed to be the case with pre-modern textual evidence, properly understood (unless deliberate dishonesty is being attributed - and this should be regarded as less - not more - likely than in modern scholarship).
So, when we read something like Geoffrey of Monmouth's - History of the Kings of Britain - we should not consider it invalidated by the inconsistencies with other sources; rather, we need to try and read it such that we are aware of different genres at work within the piece and being alert to those moments when the truth is being given...
Rather as the Bible must be read as containing many genres such as (what we moderns would refer to as) history, poetic songs, mythic stories, parables, fables, philosophy, guides for living, maxims, and so on - rather than read 'literalistically' as if it were wholly an instruction manual on a sentence by sentence basis (and therefore full of inconsistencies).
As it is, the small, clear, clockwork minds of modern secular scholarship confidently reads their ignorance and lack of sophistication into ancient works, and into the evaluation of ancient works; insensibly and uncritically - yet dogmatically; accepting its own arbitrary and self-imposed exclusions and over-simplifications as necessarily correct; such that the truth and validity of ancient writing is summarily rejected - and its wisdom and reality is hidden and/or condescended to.
And this only refers to written sources, and to those written sources which survive.
The spoken, 'oral', sources may be lost entirely or mostly, especially secret knowledge; and most importantly the tacit knowledge of the skilled crafts (or 'mysteries' as they were called - as in the 'mystery plays' performed by the guilds) - which can only be handed on by prolonged contract between master to apprentice... these are left out if we focus on surviving written texts interpreted by the latest fashions in secular (careerist) academia.
Thus, things which were crystal clear to the masters of the past are utterly obscure to modern professional historians and other 'scholars'; and - this being the problem- are 'therefore' vehemently and scornfully rejected by moderns.
As someone who was apprenticed in the guild of the ancient mystery of Medicine and also in Science, I have seen this modern cynicism prevail. Many things of great importance that were once very clear and obvious - things clear and obvious (clear and obvious, that is, to one of adequate knowledge and skill - one who has been apprenticed for many years and has learned from his masters) are now routinely (and indeed compulsorily, in the law courts) utterly denied, and re-framed as the product of socialization/ brainwashing.
Ignorance, lack of skill, lack of relevant knowledge; the dumb, numb, overconfidence of the man who knows just one thing (e.g. statistics) - such deficits and deficiencies are now dominant; because their perspective is easily explained to the majority of equally dumb, arrogant idiots; and easily rendered into formulae and rules that can be 'objectively' (but without rationale) be monitored and enforced.
To get back to Arthur - anyone who wants to know for himself the truth about Arthur, or Merlin, can never find it using the mentally-maiming conventions of modern secular scholarship - and would need to learn a very different way of reading and thinking about ancient sources than is enforced now.
And therefore he would be well-advised either to keep his discoveries/ inferences to himself - or else to refrain from arguing within the current conventions of career-effective academia - to frame those discoveries in another genre: story, song, poem, fable, parable...
Because, whatever is (if it is to be found) the true and necessary and valuable meaning of Arthur and Merlin and the rest; this truth (and it is a truth) is something or some-things excluded a priori by the methods of modern history, archaeology, textual scholarship.