In Mozart's delightfully farcical opera Così fan tutte, everyone is culpable. The title—roughly, "They're all like that"—refers to the fickleness of Woman, the hypothetical Fiordiligi and Dorabella in particular (sung by Leah Sapko and Kristina Reagle in Thursday's performance) but their fiancés Ferrando and Guglielmo (David Vogel and Jonathan Nussman) likewise prove themselves to be utter cads in the course of their very complicated attempt to prove the sisters' fidelity. The Boston Conservatory production, set......