This has been, as it should be, a very high-toned conversation, with all kinds of interesting information coming out along the way.
But back to the OP for a moment, perhaps.
From much of what has been said on this thread, I carry away the impression that several posters with an academic background want, in effect, to draw a distinction between two possible criticisms of Nibley:
1. He deliberately, consciously, and with the intention of deceiving, misused his sources so as to make them say things contrary to the evident intention of their ancient or medieval authors, by insertion, omission or partial quotation, in such a way as to favor his particular view of the past seen through a Mormon lens. In other words, he lied.
2. He was so immersed in his own idiosyncratically Mormon view of the world that somehow everything that went from the pages he read into his brain and out again through his typewriter became somehow 'Nibleyized', almost (note the 'almost') without his having to perform these acts consciously. This process might be characterized as 'taurocoprography' (to spare Dr. Shades' blushes, I have created this term to avoid an elided version of a word beginning with 'b' and containing a version of the 's' word).
I've tried to write (2) as sympathetically as I can, but as I re-read it, it seems to me simply not plausible that the kinds of distortions documented on this thread could be produced by somebody who was not clearly aware of what he was doing. So I am stuck with (1). He knew what he was doing, and he did it on purpose. He lied.
I realise that it is his writing, not his lecturing that is at issue here. But as I watch the notorious '
Horses lecture' video, even ignoring the intrusive added captions, I see somebody who seems to have no interest at all in helping his audience to weigh the evidence and arrive at a reasoned conclusion on that basis. His aim seems to be to do no more than to keep his plates spinning and to keep talking for as long and as rapidly as it takes to make the impression he wants to make - which is clearly 'Isn't Professor Nibley brilliant, and gee, it seems we don't have to worry about those horses any more.' It's more male bovine related activity, but this time the word is 'taurocoprology'.
If this man had been born in 1980 rather 1910, I don't think he would still have been in the Mormon church by the time he gave that lecture. He was too clever, and too interested in ferreting things out. The Internet would soon have seen to it that any shelf he might have constructed to keep his testimony would have snapped early on. But he was born when he was, and lived the life he lived. That seems to me to be something quite tragic - though he probably would not have seen it that way.