From Schmigadoon!, at the Nederlander. Photo: Matthew Murphy In an unsteady season on Broadway, we’ve gotten a surefire frontrunner for the award for “most musical.” Not in terms of the scale of its sets—we’ll leave that to the giant staircase in The Queen of Versailles, or whatever is going on with the flying vampires of The Lost Boys—but in terms of the most tropes crammed into one two-act stretch. If you respond to musical theater like a professor with a grading rubric, I can’t imagine you’d find fault with Schmigadoon! The show’s got love songs, patter songs, a few reprises,...