While totally accepting your point about publication not being the be-all and end-all, I think it's unfair to ask Appendix A to use a different definition of 'Eldar' to the one provided in Appendix F (which specifically excludes the population of Lorien). They're in the same book, after all!
Given Tolkien's known obsession with keeping the published material accurate, my guess is he just forgot he'd specified the Lorien elves as non-Eldar. Alternately, we can go back to your original proposal: 'Eldar' had variable meaning. Wasn't it originally applied to all the Quendi, and then purloined by the Elves of the March? I wouldn't be at all surprised if the Edain started using it as shorthand for 'Elves once of Beleriand'.
As for Imrazor... it's because Gondorians are racist.
No, really! 'Numenorean' is used as a synonym for 'Dunadan' (by Bilbo, I think?), so what the name really means is 'he was a proper Gondorian, not one of the lesser races'.
In fact, I see that UT specifically says of Mithrellas that she was 'of the lesser Silvan race (and not of the High Elves or the Grey)', so the 'lesser race' idea is right there. What's the most convenient term for 'Noldor + Sindar'? I don't know a better one than 'Eldar'.
hS