In the heat and humidity of July, which was unmitigated in tranquil Otsego county, the Glimmerglass Festival proved, yet again, that opera and musical theatre can coexist and beat the Arts-in-the-Trashcan odds in today’s society. So much of Glimmerglass’s success must be conceded to Francesca Zambello’s untiring, hands-on management and her supreme skill in selection, execution and coordination. The topic du jour in beds and breakfasts throughout Cooperstown this year is classical opera’s fate in the coming years.  Most opera houses in the U.S. are in dire straits: talk of the Fall of Opera is bruited about the Metropolitan Opera in the wake of administrative cataclysms and scandals; small, independent opera houses are clinging on with white nails suffering from dwindling endowments and audiences.