Why did Glorfindel come back to life?

[Ñólendil]

Of course, Silmarillion was edited by Christopher, and didn't he admit somewhere (I'm not sure where) that the Glorfindel thingy was his own mistake?

No, he did not. I don't see why Tolkien's essay should be discarded. It is not even in discord with the Lord of the Rings or the Silmarillion as published (which holds considerably less authority than the former). The 'Glorfindel' text is an essay which makes the "two" Glorfindels of The Fall of Gondolin and The Lord of the Rings to be the same, and Tolkien remarked that it improved the story. He wrote some time after 1968 and he never went against the decisions he made in that essay. I think it should be accepted.

"I know there is a lot of interesting info in the additional collections of Tolkien's writings, but I look rather askance at them because he didn't integrate that material into his published writings (Silmarillion aside)."

J. R. R. didn't or Christopher? If you mean the former, what material are you referring to? Where in The Hobbit, The Book of Lost Tales, The Adventures of Tom Bombadil, The Road Goes Ever On or the Lord of the Rings should Tolkien have put his Glorfindel essay? Where should he have said that the "two" were one and the same? I don't understand your logic. Nothing in the Peoples of Middle-earth, in my opinion, should hold as much authority as anything published during Tolkien's life time, but that's no reason to "look askance" at them.

Again, I don't think the re-emergence of the reincarnated Glorfindel, or whether Glorfindel of LotR is the same as the Silmarillion Glorfindel, can be definitively resolved, neither through the writings published while Ronald was still alive and re-editing, nor through the post-mortem collections of his discarded and unincorporated works.

Why do you believe this? I take the opposite view: there is a very clear, coherent essay written by Tolkien late in his life that explained the whole deal, and the man never went against this essay as far as anyone knows. As far as I'm concerned the whole issue is definitely resolved.

There is nothing in The Silmarillion or the Lord of the Rings (the latter here being much more weighty than the former) that would suggest there were two different Glorfindels. There are two texts that shed light on the subject, they are found in the Peoples of Middle-earth, and both say that they were the same Glorfindel.

the History of Middle-Earth series because that series is more of a collection of (with exceptions) unfinished or discarded versions of what was covered in the LotR and Silmarillion (along with some other material).

This is entirely incorrect. No offense, but I don't think you've taken a close look at some of the HoMe books. Drag out the History of the Silmarillion and Peoples of Middle-earth. You'll find that many unfinished and discarded texts, and many unrelated texts found their way into the Silmarillion as published via Christopher Tolkien. The Doriath chapter was nearly completely the work of the son. You'll read Christopher beat himself up a lot about the stuff he put in The Silmarillion, a couple of times he even wonders if he should have tried it. In PoMe you'll actually find material that was omitted from the Lord of the Rings by accident. The entire primitive mythology of the Silmarillion as published was "discarded" by J. R. R. Tolkien in favor of a scientific one.

The Silmarillion is nice to read, but I think most take it way too seriously. When you really get into studying the texts you'll find that the more you know -- the less you know. "Canon" is not an easy thing to find.