Thanks for reading.
Anyway. On the secret origin of Sir Lancelot du Lac.
Le Morte d'Arthur doesn't describe Lancelot's arrival at Camelot; suddenly when the war with Rome comes up he's just there. In the Old French Vulgate cycle (one of Malory's sources) Lancelot's birth and youth are described after Arthur and Guenevere are married and Nimue has learned all Merlin's magic and trapped him in the cave. (Which suggests that Lancelot is considerably younger than Arthur and Guenevere. In The Once and Future King Arthur is about eight years older than Guenevere and Lancelot, which "at the age of twenty two [when Guenevere and Lancelot meet] ... seems the verge of senility". Here in AKOTAS they're all three within less than a year of each other.)
In the Vulgate after his parents die and his father's lands are invaded
Lancelot is fostered by the Lady of the Lake (I'm unclear whether this is
Nimue or a different Lady), who then brings him to Camelot and is one of two
major ennablers in his relationship with the Queen (if she's Nimue, that'll
be because she now has Merlin's gift of prophecy). In Le Morte
d'Arthur and The Once and Future King, Lancelot's father King Ban
is still alive after Lancelot comes to Camelot: sometime after the Roman war
Ban calls on Arthur to war on Ban's enemies as Ban did for Arthur at
Bedegraine. In The Mists of Avalon the Lady of the Lake - not Nimue,
the one who is killed by Balin - is Lancelot's mother, conceiving him
during an Old Religion king-marries-the-land ritual. In none of these does
Lancelot ever ascend to his father's throne.
Arthuriana sources I use or recommend:
Arthuriana - the Journal of Arthurian Studies; the website of the quarterly journal of the North American Branch of the International Arthurian Society.
The Camelot Project at the University of Rochester.
Camelot In Four Colors: A Survey of the Arthurian Legend in Comics
Mystical-WWW - The Arthurian A2Z knowledge Bank which has encyclopedically-arranged entries on the characters of the Arthurian legends.
Le Morte Darthur: Sir Thomas Malory's Book of King Arthur and of his Noble Knights of the Round Table, Volume 1 and Volume 2.