I agree with you for Luna, although I think her craziness is also a way to mask the secrets she has, as, I believe, she didn't want to talk about her necklace probably for one of those reasons. That, and all the shit she's seen and done, she is a high ranking mafioso after all. As for Gracie, I kinda agree with you, although I don't know if it's actual insanity, and perhaps psychopathy, or if it's just the fact that she's so studious she doesn't understand emotions, and so, with that, she might not understand certain things about morality. Like, I honestly think she MIGHT have been the one to have poisoned us, or been asked by whoever is actually trying to kill us to poison us to see what it would do.
Either way, I think we can all agree on one thing. Luna best girl.