My guess is that the pale tree and the sylvari are minions of the jungle dragon. But the pale tree was able to break free from it like poor Glinth did. I guess she use the dream to free the sylvari from the dragon. The Nightmare Court are sylvari who were touched by the dragon and became the original dragon minions. That’s why they could not be saved. And that would be an answear to the question, why they could not be corrupted by other dragons.
My other theorie is that the sylvari and the pale tree are the answear of Tyria itself to the dragons. But this only based on what i saw in the Eternal Alchemy.