The core issue I see here is you have a personal compulsion to need these things to be answered. This is your personal problem, not a problem with someone making a suggestion. If it doesn't make sense in your internal logic, then that's for you to deal with. It's not the problem of a person expressing an opinion of what they want to see.
If I said Demon Hunters should have Horn and Wings options if we get playable Demon Hunters, then I am expressing an opinion, making a suggestion. I don't need to explain HOW they get the Horns and Wings options. If I want to see Horns and Wings on DH, then it's just something I want to see for my own reasons. If there is no lore to explain such a customization option that is not my problem
Using this example, I will explain why it's meaningless to use lore to support what SHOULD be done. It's all a matter of perspective.
A: - I am pro-Horns/Wings, I see lore having Illidan set a precedent for all Demon Hunters, and thus it's an acceptable suggestion
B: - I am anti-Horns/Wings, I see Illidan as a unique case, and thus no regular Demon Hunter should have access to Horns/Wings because there is no lore to support them having them.
They are both subjective interpretations of the same lore. Are Horns and Wings exclusive to Illidan in the lore? It all depends on what you personally think it
should be. If you don't like it, you will use it as an argument against the suggestion. If you like it, then you will allow the lore to support it. This is why I say it's very very meaningless to use lore to support any argument.
Lore can absolutely be twisted to support any position, depending on how you feel and what you are willing to accept as legitimate lore. All the while, anything you disagree with? You can invoke 'Blizzard can retcon' and dismiss any argument you wish, because you can't give Demon Hunters Horn and Wings options without addressing the existing lore to clarify the ambiguity. Or you could do what Blizzard does, and simply not explain it and don't give two fucks about the lore.
There is only ONE way to regard lore, and that is in retrospect. If talking about any suggestions or future propositions, then you can't regard lore as an immutable factual record. It doesn't work like that at all. All you can do is manipulate lore to suit your argument.
This is why the banshee taking their bodies example is absolutely pointless to discuss. Whether they can take their bodies back or not is basically up to how you feel about it happening. Should it happen? Should it not happen? Purely subjective. We are able to make an argument both for and against it using existing lore. End of the day, it will not be relevant to whether we should or should not get Undead Elves. In my own opinion, using lore as a supporting argument is no different than using a thinly veiled excuse to dismiss something you don't agree with.