Wow, that's a lot of information there... I have a friend called
Kayleigh and she thought that it's from Gaelic or something... I have a feeling that some name sites attribute it to the root Ceilidh (a Gaelic word for a sort of traditional party/gathering, when everybody dances and sings songs). I think, though, that it most likely springs from the modern popularity for the elements "
Kay" and "Lee"/"Leigh". :)