Is Albert going to collapse when he realizes that it's all true and his father is actually a total scoundrel? Oh dear. AND - more importantly! - does Mercedes know???

I think Mme de Villefort must be the poisoner, simply because who else could it be? We know it's not Valentine, it would be weird for it to be some random servant we haven't met before, and Villefort seemed genuinely shocked to hear that there had been poisonings in his house.

I suppose there's always Valentine's creepy half-brother. But he's like nine, so all this poisoning might be somewhat beyond him, and anyway he wasn't the one questioning the Count about poisons.