As someone who has often spoken in favor of implementing playable High Elves, I find your entire post absolutely agreeable. I, and many others , have in the past posited that they (High Elves) would likely be physically altered prior to their implementation -- in an effort to create distinction between them and Blood Elves. That is exactly what Void Elves are. Their physical divergence is totally acceptable to anybody who understands how central faction identity is to the game, overall.
The issue, for me, is entirely a matter of narrative peculiarity. If we're understanding what has been datamined correctly, then the TL;DR for Void Elves is as follows:
- Magister Umbric, as well as his cadre of supporters, begin to study the Void. They're banished from Quel'thalas.
- While seeking to convince the Blood Elves to re-join the Alliance, and promptly being banished from Quel'thalas herself, Alleria learns of and begins tracking Umbric.
- Alleria catches up to Umbric on a Void-infused world, where Umbric has unearthed an ancient artifact containing untold Void energies. Shortly afterwards, perhaps upon opening said artifact (a cube), Nether-Prince Durzaan appears and claims he's been waiting for the perfect vessel(s) to empower with the Void (presumably, based upon his statements, he intends to create more Ethereals) and proceeds to envelope Umbric and his comrades in raw, Void energy.
- The sudden influx begins to radically affect their physical forms, infusing them with Void and ultimately changes them into the Void Elves we now know before we can fully interrupt the process.
To me, that scenario reads as absolutely plausible (in-universe). The part that I find odd, from a narrative standpoint, is the complete lack of interplay with the actual High Elves (as in, Silver Covenant) that Alliance players have fawned over and for whom the Void Elves were presented, presumably, as a compromise.
A very simple fix to this, which wouldn't require any re-writing that I'm aware of, would be to add a conversation between Vereesa and Alleria (to be exchanged immediately following the initial quest you grab in the Stormwind Embassy) wherein Vereesa insists that the Silver Covenant accompany Alleria on her sojourn to Quel'thalas. It would be quite characteristic of Vereesa to try and flaunt herself in front of Blood Elves, as she likes to do, and would be agreeable to Alleria as her purpose for going to Silvermoon in the first place is to recruit the Blood Elves back into the Alliance.
End that conversation with a "don't speak, unless spoken to" from Alleria, directed at Vereesa, and you've just created a scenario wherein all of the Silver Covenent is present for the "Voidening" and yet you haven't really done any work to make it believable. Win, win, win on all fronts.
EDIT:
This just came to me as an afterthought, but if Alleria's sojourn to Quel'thalas included Vereesa (as well as only her most staunchly pro-Alliance militants), this could be a good way of integrating what is left (i.e. Silver Covenant who then, in this scenario, remain "pure") back into the fold of the Blood Elves -- which then could allow for the Blood Elves to receive blue eyes as a customization option, while leaving very little room for fuss. This whole ordeal, as presented above, would conclude the "playable High Elf" discussion forever... but in a way that doesn't simply write off the fact that these people have fought, front-and-center, with the Alliance for 10+ years.