The parentage of
Mordred is a later concept. In
Geoffrey of Monmouth's
History of the Kings of Britain, he is the son of
Lot of Lothain and Anna, full sister of
Arthur and daughter of
Igrain and
Uther Pendragon. (Geoffrey lists no children for Gorlois.) He is then full brother to
Gawain and full nephew to Arthur. Later legend brings us Morgaws and Morgan, with a third sister, sometimes named Elain.*
Morgan le Fay is undoubtedly a euhemerized goddess, likely derived from Medb/Mab, Morrigan, and Modron. She is only called Mordred's mother in later fiction. Her two sisters may be doubles of her, and this is my reasoning: the Celts often had triple gods and goddesses who were also the same god or goddess--not unlike the Christian idea of the Holy Trinity. Brigid is a triple, as is Lugh. It is not unlikely that at some point, a historical woman named Anna was written out, or confused with the tripled goddess.** Morgan is said to have stolen Excalibur, according to Sir Thomas Malory (and hense probably the Lancelot-Vulgate cycle, though there does not exist an English translation for me to check that, and I do not read very much French).
Regarding Guenevere/Gwynhafar, this may be another case of the triple goddess. Gwynhafar may be a confluence of historical queen and Earth Goddess, a personification of the land, like Eriu and Penardun. As such, it is logical that there would be three of them, just like Eriu and her two sisters. As for the violation, that is probably more Persephone myth than historical event.
The affair between Lancelot and Guenevere is not an invention of the Victorians--if anything, they tried to expunge all versions of the tales containing this bit of information. The earliest account of the affair is in Chretien de Troyes' romance Lancelot, or, The Knight of the Cart, which dates to about 1174. He may have been influenced by the Tristan and Iseult tales, as he is said to have written a version which is now lost.
Merlin is a whole other subject, an amalgamation of historic bard Myrddin, legendarly wildman Lailoken, and medieval Antichrist. I won't get into him here, nor Vivian/Nimue.
The problem is with books like The Mists of Avalon, which uses little scholarship and a lot of agenda, ignoring the old texts so as to tell a fun little story that will make them a lot of money. I hate when this happens--it's like watching Disney's Hercules, and having to explain to children how they got it all wrong. A few years ago, my family watched the Merlin miniseries, and I had to explain to my parents how they got the whole thing wrong. Me, I always go to the oldest texts I can--in this case, Welsh and Latin. The medieval romances are true though not factual--fun, with some mythological or psychological truth, but it didn't happen that way.
*Elain is such a common name in these stories, I think it's like calling a man Jack--when you have no name, substitute with "Elain."
**I am not talking about the Triple Goddess of neo-pagan lore, either. That has an emphasis on "maiden-mother-crone" which the Celts didn't have. The triplets were true triplets, but with differing aspects: for Brigid, it was Smithing, Fire, and Poetry.