Valedhelgwath posted I admit there is subject matter concerning Glorfindel's reincarnation, in fact a lot of it, but to me none of it is really conclusive. If the Third Age Glorfindel was to be accused of the murder of a balrog back in the First Age, and the prosecution used as their evidence what is written about his reincarnation in HOME, in a British court he would be aquitted. There is insufficient evidence to PROVE the case.
What you appear to mean by 'prove' and what Virumor
probably similarly means by 'definite' is really about what Readers accept as a given however: that unpublished texts (not published by the author) are indeed unpublished, and thus are not 'final' in this sense. If not, what else do you mean? But this basic stamp is not the only consideration of course, as once competing unpublished texts go up on a scale there are possibly other factors to tip those scales, other considerations that show equally 'not final' texts (in the sense I think you mean), are yet not equal, or not of equal weight, in other ways. And here, there are other factors.
To say it another way: one can 'prove' both issues here about as much as is possible within the limits of the situation, and beyond that one is swimming in general waters. What Tolkien would have decided about the matter in this 'final' sense cannot be known, but that doesn't change what we do know; that doesn't change what he did at least put to paper.
Much of what is published in HOME are Tolkien's early drafts and notes, and reading through them, you can see Tolkien frequently changes his ideas and story lines. The question is, what do we believe to be the truth?
But there's a lot of material that is not early as well, and Tolkien still made changes in post-Lord of the Rings traditions of course.
Some people quote sections from the Lost Tales, and insist on using the names from those books. These are Tolkien's earliest drafts, however, and much adapted over time. Tolkien clearly did not intend what he wrote in those early texts to take prescedence over his later writings. So, in that case does his latest writing take prescedence over everything else? You would think so, but if that is the case, some of the last writing Tolkien did was to rethink his whole Creation story of the Sun and Moon being born from the fruits of the Two Trees. In his last pieces of work, Tolkien was creating a universe more akin to our own. So do we tear up the chapters in the Silmarillion about the creation of the Sun and Moon? I hope not, because they are beautiful concepts - far better than what he was rewriting them as.
This could be an interesting thread, but just briefly here, no I don't believe one need tear up those chapters.
Now back to Glorfindel... In The Peoples of Middle-earth (...) The gist of these notes makes it clear that at the time of writing Lord of the Rings, Glorfindel of Rivendell was not conceived as the same character as Glorfindel of Gondolin. Tolkien says, 'Its use [i.e. the name 'Glorfindel'] in The Lord of the Rings is one of the cases of the somewhat random use of the names found in the older legends ... which escaped reconsideration in the final published form...'.
As the Encyclopedia of Arda would suggest too
However they fail to mention Tolkien's interesting 'Glorfindel tells of his ancestry in Gondolin'
noted by the author at the time The Lord of the Rings
was being written. That might be only suggestive but there is something about this I find quite likely: that is, after 'somewhat randomly' choosing this name (even if this suggestive note did not exist), it's very likely that at some point in the years
it took to finish the book, JRRT was going to connect the name with the already existing character, even if briefly in his imagination.
What he did not do at the time (it appears), was sit down and consider the matter in detail and on paper, as he would do much later, and ultimately find out the 'truth'.
On The History of Middle-Earth
in general, the '77 Silmarillion was constructed out of what is found in the twelve volume presentation. To phrase if differently, HME isn't 'added' stuff, it's all the stuff, or nearly so. Very generally speaking the legendarium is a collection of unfinished, disparate texts and traditions written at different times -- ranging from a hard to read (or brief) marginal note, to the only complete and 'finished' version of the Silmarillion tradition that ever existed (the Qenta Noldorinwa
'The Silmarillion', again in the widest sense, is very evidently a literary entity of a singular nature. I would say that it can only be defined in terms of its history; and that history is with this book [HME XI] largely completed (...). It is indeed the only 'completion' possible because it was always 'in progress', the published work is not in any way a completion, but a construction devised out of the existing materials. Those materials are now made available, save only in a few details and in the matter of Túrin...' Christopher Tolkien, Foreword, War of the Jewels
HME contains the Silmarillion tradition, or 'Silmarillion proper', itself a part of an imagined larger collection. The nature of the HME style of presentation allows for a much truer picture to be drawn, giving the texts as Tolkien actually wrote them and left them at his passing, or maybe Christopher Tolkien might attempt to describe an extant text rather than reproduce it.
By its nature a one-volume presentation which attempted a measure of consistency, like the '77 constructed Silmarillion, will naturally exclude competing materials that might be considered to destroy too much in the way of believability. Not all competing details need be excised however. Tolkien was concerned with consistency and a certain measure of 'intentional inconsistency' (or seemingly conflicting details). Some differences would be expected due to the circumstances; in short 'too consistent' is not desired, and too much inconsistency would begin to break down the desired effect.
Importantly, Tolkien was a world class niggler: sometimes it seems
as if he could not sit down on a given day without revising something, or without a new idea popping up and spreading like fast growing vines. Nothing wrong with that in my opinion, and while this may seem to help the 'not final' argument, again, we cannot know what we cannot know.
I have no problem with someone who posts 'I prefer this idea or that', I have my preferences too, but there are factors too notable for me to ignore with respect to some issues. One can 'believe' what one likes of course, but I think most people are already heeding something that's only natural when dealing with the work of an artist (who worked on something for much of his life) with a penchant for revision: that some things, although written, or 'once written', were really never meant to be seen by Readers as far as the author was concerned -- they were to him 'old' ideas not really true anymore. And we only became aware of them because someone else made them public. Some issues might indeed be cloudy...
... but in my opinion these two conclusions (now under discussion) are about as clear as a Tolkien Reader can find.