Having already seen and reviewed the musical “Avenue Q” in its New York premiere — where it won Tony Awards for best musical, best original score and best book — and again when it made its Chicago debut and, most recently, in a summer stock production in Saugatuck, Mich., I wasn’t particularly concerned when my scheduling made it impossible to attend its opening on April 23 at the Mercury Theater, 3745 N. Southport Ave. But when I saw the production had been extended again and again from …