A Philadelphia production of “Margaret Garner” seems to get worse with every performance. “Is it possible for an opera to deliver experiences so radically different without any significant revision of the score, change in cast, or alteration of production? That happens with Broadway musicals. But opera is supposed to be sturdier – and transcend things like the struggling orchestra heard on Feb. 24. Or can an opera be so eager to succeed with a mainstream audience that it lacks the anchor needed to overcome a performance that, for any number of reasons, loses its way?”