What real life person, persons, or events was Morgan le Faye, cast as an evil enchantress in most King Arthur tales, based on?
It seems that she is a stereotypical fictional character, mainly a female sorcerer or goddess, based on early Celtic and Welsh mythology, poems and legends. The character isn't based around any particular person from what I understand.
You can find more info about the character here: Morgan le Fay
The fact is scant historical records survive from the time during which a historical Arthur, if any, would have lived, and usually, any attempt to determine a historical truth behind a character or event from such legends is left to guess work.
It certainly does seem to be the case in some situations that events described in the legends of King Arthur were based on some older knowledge, now lost to us, of events which did occur. The historian Geoffrey Ashe, for example, has identified the king Riothamus as a king who ruled Britain at about the right time, whose name meant "high king" (and Arthur was thought of as a "high king" in some contexts), and who marched on Rome, just as Geoffrey of Monmouth recorded Arthur doing (a major difference being that Riothamus never came back). Ashe has suggested that the missing piece of information here could be that Riothamus' personal name may have been Arthur.
As far as how all of this can be applied to Morgan le Fay, all we have is guesswork, and some very imaginative, at that. Most attempts to trace her character have focused on identifying her with triple goddesses that were worshiped by the ancient Celts. Some historians, though, (albeit not mainstream ones) have suggested one theory which attempts to suggest that a real person could have been behind the character of Morgan le Fay. These historians point to older, pre-Roman traditions whereby Welsh married women were allowed to take on multiple lovers, and they have used this to argue that later, Christian writers misinterpreted this as adultery in their retellings of the tales. These historians claim that Guinevere would not have betrayed Arthur by becoming either Mordred's mate or Lancelot's, but was only practicing a custom in which the wives of powerful leaders mated with their husbands' allies in order to ensure loyalty. Furthermore, Mordred would not have been Arthur's son and nephew, but only his nephew. This, they argue, was a misunderstanding based on the fact that, since the true father of a child could not be known under this system, all wealth and status was passed down matrilineally. That is, Mordred may have been Arthur's nephew who succeeded him, and later Christian writers, seeing the references to Mordred as Arthur's nephew, were unable to resolve this, and took it to mean Arthur's son was also his nephew.
It's all an interesting theory, but lacks any real evidence to support it. All we know for sure about Arthur and Mordred is that the Annales Cambriae records that Arthur and "Medraut" both fell in the battle of Camlann in 537. There is no indication that "Medraut" was Arthur's enemy, or that he succeeded him. For that matter, there is no indication that Morgan le Fay was a real person. Anything we have is scholars' attempt to make sense out of the fact that certain aspects of the legends seem to draw on some older traditions which are now lost to us. Anything beyond that, is speculation.
Though I lack a link for you, my reading of grail histories has always implied that Morgan was a characterization of Lilith.
Interestingly, the Old English word 'morgan' derives from the Germanic, relating both to "morning", and to "tomorrow". As such, 'morgen' is possibly a light-bringer meaning - which would parallel such name with the strengths gained by those who overcome one who has such name (e.g. Lucifer).
p.s.; (quaintly enough, the Dutch use the same work as a unit of measure, based upon the rough amount of land that one can farm during the length-of-time that is a 'morning').