AFAIK most of the Sunfury's aren't Fel Elves, just the few in his inner circle who were allowed to OD on Demon Blood by Kil'jaeden. They've totally committed themselves to being demons and worshipping KJ that they are way beyond redemption. There is absolutely zero chance that Silvermoon would reintegrate an organization that still openly allies with the Burning Legion, especially not the new Sunwell restored Silvermoon.
The difference between a regular green skinned Orc and a Fel Orc with demon blood in their veins is literally the difference between the genocide of an entire alien race. Fel Orcs won't work as a subrace because the nature of Fel-anything is nothing more than "worship legion, kill everything non-legion". Hell, I don't think there is a single instance of an entity that serves the legion being friendly to us.
Fel Corruption doesn't really work in the same way other corruption does. For example, the San'layn would work because they were undead under the service of the Lich King, and we know that it is possible to break free from the Lich Kings grasp as evidenced by the Death Knights and the Ebon Blade, among others, and perhaps even more so now that Bolvar is in control. Sylvanas would very likely adopt the last remaining Darkfallen into her Forsaken, and reintegrate them back into the Horde. They don't have to be Silvermoon elves.
Fel Elves and Fel Orcs don't really have that liberty, for all intents and purposes they are straight up demons who now only resemble the races they used to belong to.
The comparison to Highborne doesn't really work either. While the Highborne totally dicked the dog and blew up the world, Arcane magic in itself isn't corrupting in the way Fel magic is, and the Highborne were in hiding for 10,000 years before they even mentioned being reinterated, and even that was met with heavy resistance. I don't know where you are getting the Highborne feeding off demonic energies came from, those that did did not remain Night Elves, they became Satyrs.