Taz'dingo. Heads-up warning, this post will probably be long. I'll put the TL;DR up here for you lazy guys, but if you want to discuss it, please read the whole post.
Also, this is mainly thought of as a suggestion as to how the various beliefs, namely those of Trolls and of Night Elves, could fit together. It was not created based on anything, but simply because it would work as a missing link between both theories.
TL;DR: Dark Trolls were transformed by Well of Eternity into new beings. These beings were able to see Elune in her original form[Night Elven form] and started revering her.
They then tried to turn into Elunes likeness, succeeded by using the powers of the Well of Eternity and effectively became the first Night Elves.
Okay, let's start the long part then.
Troll and Night Elf theories:
In the Trolls belief, the Night Elves were created from Dark Trolls. After the war with the Aqir, a group of Dark Trolls pioneered towards the Well of Eternity and, according to lore, were turned into "beings of immense power". The Trolls belief that these beings were the first Night Elves.
They consider skin color, the fact that they have about the same height, build and similar ears, as proof, that this theory is correct.
It is supported by the statement, that Night Elves have their origin in a species of "a primitive tribe of nocturnal humanoids"; though it is not clear whether Dark Trolls were actually nocturnal, the other 3 parts of the description would fit them.
In the Night Elven belief, their origin is not linked to the Trolls. They are convinced that they were either created from Elunes offspring or by Elune herself, similar to how Christians believe that God created man.
Proof, in their eyes, are the similarities between Elune and the Night Elves. They look pretty much identical and Night Elves also share the non-animal parts of Dryads and Keepers of the Grove.
Missing link:
As there was no evidence for either theory, I thought that maybe both could be right. Hence I was looking for a missing link that would be able to connect both theories and, by chance, found it recently. Keep in mind though, this is only something that could work, there's no evidence behind it and it hasn't been mentioned in the lore either. I may not be the first one to have come up with this, but I doubt there've been more than a handful.
If you've read the TL;DR already, you know what I'm heading for, but let me state it again: I'm talking about ascendation.
My theory connects both versions, namely in the sense that it were originally Dark Trolls, who got transformed by the Well of Eternity and thus came to power. This part is proven in lore.
Because of the power they gained, they were able to catch glimpses of Elune, similar to how Paladins see the Light, Shamans can see Spirits and the like. When they saw Elune, they considered her a goddess and started revering her.
After years of praying, using the powers of the Well of Eternity, they were able to ascend and achieve the physical form of their god, Elune.
There were two things that actually made me think of this:
a) Twilight Ascendants. http://static.mmo-champion.com/mmoc/...ascendants.jpg
From what I think, they were members of the Twilight's Hammer clan who managed to achieve enough power to actually transform into Elementals. [Though it does not look like one of the better transformations, but oh well...]
b) Shadow Ascendants. http://www.wowwiki.com/Shadow_ascendant
The Shadow Ascendants are Forsaken who managed to leave their undead bodies behind and transform into living shadows.
Well then, that's my suggestion for the Night Elven origin. Please feel free to discuss this as you like, new suggestions are also welcome.
Thanks for reading, and may King Rastakhan feed on your bones.