I haven't caught up on the thread yet, but there is a really simple reason why the dragonborn wings are spectral and made out of energy.
Because DnD players are pendants who can never let a small detail pass without making a huge deal about it.
The immediate thing that would happen as soon as a Dragonborn Fighter attempted to grow their wings? "Well, what about your armor?" So then every single dragonborn would need to have specialized armor built to allow for their wings, which would mean that any armor not modified would lock their flight ability, or if an enemy tied them up with ropes they wouldn't be able to sprout their wings, or if they... and on and on it would go.
So, the wings are made of energy. They allow the dragonborn to fly while ignoring things like clothes and armor. Because to do otherwise, DnD players would make a mountain out of the inconsistent little molehill.