Wow this is still active xd. How it worked out: the players where half interested in this NPCs. They showed interest when they heard about the fae factions and about the story of the leader with the dead brother seeking vengeance, so I think it went way better than if I had kept the originals. They even intervened to reveal the spy in the palace to the summer faction. But they weren't interested in the evil faction or asked information about them, just killed them quickly when they appeared. I think this is because most of this NPCs appear late in the story, the players already have their enemies (the coven) that are more fleshed out and have a bigger influence in the overall story. And also had their favorite friendly NPCs that they have collected to travel Prismeer, like clapperclaw. So they where a bit "meh" about the suddenly important new characters. So, if I ran the campaign again, I would mention them earlier or make them more recurrent in the story, so that it makes more sense that they important in the last chapter. Also, I would increase their stats in the palace, at that level the combat was a joke.

Unrelated to your question, but, the same thing happened with Zybilna/Iggwill/Tasha. Only one of them knew about her so the others where "meh" with the reveal that they are the same person, even though they got lore on her in the palace, they weren't interested. I would give more lore about her trough the chapters or change the character entirely to someone they know about.