[Lalaith]

I quite agree, this is all making my head spin!
But one thing on which Tolkien does seem to be quite categoric - the definition of Eldar is 'Elves of the Great Journey.' Those who refused are Avari.
BUT - while all High Elves (Calaquendi) are Eldar, not all Eldar are Calaquendi. The Sindar and Nandor, who made the Journey but never got to Valinor, are Umanyar - defined as 'Eldar who are not of Aman'.
BUT - both Umanyar and Avari are Moriquendi - 'those who never saw the light of the Trees.'
I'm not sure if this helps.