Was it? How do you know she couldn't transform already, but chose not to? There's a myriad of possible reasons for that, alone. Her "transforming into void form" at the end of that cinematic can also be explained by saying the "fallen naaru"'s energy was too much for her and she had to take on void form to properly contain it.
You're taking incomplete information and filling the gaps with headcanon, and treating it as official lore.
The point is none of that is specific void elf lore development. It's just simple stuff that any other mage could do.And yet a Kirin Tor mage didn't do that, it was a void elf who did that. That doesn't take anything away from them.
For the purposes of the story being told, what would be the difference from a mage teleporting those Horde soldiers into the bottom of the sea... or into the belly of an active volcano? None. Again, none of that is void elf lore. There's no development to their lore. We don't find out anything more about their origins, how they came to be, how more void elves are created, they didn't learn new things to do with their powers, nothing.Also, they used their powers to send enemy troops into the Void, something that no mage can do.
Apples and oranges. Orcs already had truck-loads of development, and they had their "come back" in the form of the orcs of AU Draenor fighting the demons. And the journey to Argus was a draenei expedition, in which they allowed champions of the other races to come, not whole armies.Besides, I really don't see why it's a huge deal that void elves don't appear against N'Zoth, when orcs and blood elves didn't do jackshit against Kil'jaeden and in the invasion of Argus.
We saw how a trap created by the void lords got interrupted haphazardly in the middle. I honestly doubt they'd use the same process-- a process which, again, we have no lore about other then (insert Admiral Ackbar meme) "it's a trap!"-- that could potentially kill the other elves.You literally saw how some void elves were created in their recruitment scenario, and furthermore they were also introduced prior to the expansion, as they are marketed as a Legion allied race. Their recruitment happens at the end of Legion, not at the beginning of BfA.