First of all, do you think that is too hard for them? The CR ratings are for 3-5 people, so I picked a slightly higher CR.
Secondly, if someone dies of acid, can they be revived by normal raise dead spells, or does it require something more extravagant? So far, they have known to run before dying, but y'never know, really, and they've put so much effort into their characters' backgrounds, I'd hate to kill one without a chance of revival...but I certainly want the dragon to be a big challenge, as dragons are supposed to be.
Thirdly, if a dragon eats a character (we have a fairy, which is why I ask), do you rule the person completely dead?
Also, would it be feasible for a young adult black dragon and adult green dragon to have neighboring lairs? I read somewhere that these two dragons often form crude alliances. Would they share the same lair? This is relevant to the above topic because NPCs of higher level are supposed to be fighting an adult green dragon while the PCs distract the young adult black dragon ally.
And lastly, any advice on making a unique dragon's lair? Draconomicon has been helping me a lot, but I'm always open to suggestions.