[Galin]

It [L&C] is variable and certainly not definitive. It is not canon in the sense that Sauron created the One Ring and Gollum destroyed It and himself in Mount Doom.

I agree. And 'canon' for me is author-published stuff. That doesn't mean I ignore the legends of the Elder Days of course, but that's a more complicated issue.

What you have to understand about LACE is that (per C. Tolkien who has some weight or doesn't have weight depending on the lunar cycle) it was written by men and not the Eldar; therefore it is a retelling of ancient lore, so there is an accuracy issue from the start, considering what we know about chroniclers adding or deleting information to suit their needs and prejudices. One only has to look at Beowulf, a poem Tolkien cherished, to know that it was a pagan tract rewritten with a Christian patina many years later and not by the same hand(s).

That's true but I'm not sure why Elfwine would necessarily be considered wrong about a given something, unless we have more of an internal reason to question him. I mean, what you say is arguably built in, I guess, but when it comes to specific ideas the reader, if told 50 or 100 let's say, would probably take this as 'the best he or she can know' -- again, unless something else internal shakes the notion somewhat.

The Eressean orc-origin idea was internally delivered as a belief for instance, but should the reader question the Wise of Eressea? And even if we did, if Tolkien himself had published this 'origin' alone, we might wonder a bit but in the end would still probably be noting this origin as 'fact', at least as far as what appears in the 'true' legendarium that is.

In general, Elfwine was supposed to get his information rather directly from Eresseans, which is actually the 'more reliable' [in my opinion] transmission compared to the later idea.

I'm going on memory here, but I think the role of Elfwine possibly 'survived' in text until the later 1950s, around the time of L&C. Certainly The Lord of the Rings first edition was published before this of course, but I would have to check the first edition 'transmission references' compared to the second edition to say anything possibly meaningful about that.

That said, and not that you said or think otherwise, but the loss of Elfwine might only mean we need a new 'internal author' with some niggling to reflect this...

... but that said too, there seems to be notions within L&C that were revised, or arguably were, or are at least a bit questionable. For example, even the naming customs appear to have been revised according to Christopher Tolkien [compared to the shorter text on naming from the later The Shibbolerh of Feanor]. But I wonder if the 'missing' Chosen-name of the Noldor, for instnce, is only missing due to brevity, and the consideration that it doesn't seem to be a custom among the other Eldar in any case.

But certainly I agree that L&C awaited revision if it was to become part of the author-published legendarium.

Finally, as nearly as I can tell, Tolkien never adjusted anything timewise in his chronologies to account for changes he made according to LACE, nor does any quotes I've seen from either Galin or Cellardur indicated a fixed time for maturity of Elves (or, more importantly, Half-elves); in fact, Tolkien bounces back and forth throughout his life from something as outlandish as 3000 years right down to the same as mortal men.

There is a note from CJRT that seems to say he thinks his father means, at one point anyway [from something in a text], that one can just 'plug in' 144 Sun Years and employ the old dates that were witten with the much smaller ratio [almost 10] in mind.

And while that works to drastically lengthen the years concerned, for instance from the Awakening of the Quendi [which gives much more time for 3,000 years to maturity to begin to dwindle] to the Rebellion, it just doesn't seem to work in all places, as the flight of the Noldor would take notably long, if I recall correctly.

As Galin pointed out, the last mention from Tolkien indicated they aged relatively the same.

Well with Tolken one never knows

But I would be surprised if anything turned up about Elven ageing that could certainly be dated after Eldarin Hands, Fingers and Numerals.

At the moment I'm not sure what text is later than the other: the one that represents 50 [some 100], or 3,000 but dwindling in Middle-earth; or the citation from the Line of Elros actually. I'm not sure it's possible to even know between these examples, but I haven't looked at the external dating recently.

By the way I do remember it was someone at Barow Downs who opened my eyes to the possibility [and in my opinion probability] that the notion was 3,000 and dwindling in Middle-earth...

... it was years ago now, but thanks to that person