Feanor Self-Important?

[the phantom]

But if this interpretation of the line in question is correct, what are we to make of Mandos' response: "Not the first"?

Well, that's not a question for me to answer because I personally don't think Feanor meant that when he called himself the first. Someone else brought up the idea and I just gave 'em some book quotes.

My primary concern with the timing of Feanor's birth is this- did the idea of him being the first-born Elf make it to any later versions or was it refuted- or neglected perhaps?