From my perspective, as a player, what conclusions can I take from this story?:
1)after Arthas annihilated 90% of HE, 10% survived and were led by Kael'Thas who; under Illidan's influence, mark a divisive point in the history of his people.
2)A dictatorship was established in Silvermoon in which; as a HE, if you did not follow the vampiric practices of Kael'thas you would be exiled from your lands, which resulted in the exile of 1% of that population. This 1% returned to the name of HE while the remaining 9% adopted the name of BE.
(up to this point only a political difference has been established between both groups)
3) From this moment, BE begin to drain arcane energy, using vampiric magic
(arcane/fel) (which was taught by Illidan), they even took burning crystals (full of fel energy) to experiment with them, reestablish Silvermoon and drain their energy, this caused a series of phenotypic mutations in BE
(it is here that the first phenotypic change is established)
4) When the sunwell was restored, the corruption created by fel magic in BE's bodies was eliminated (not the mutations)
5) Meanwhile HE didn't change.
You can clearly see the political and phenotypic differences.
The point of physiology remains irrelevant, since the physiological differences between HE / BE and the other races were never explained to us, nor any in-game physiological principle.