Already an improbability, the notion of a Mets collapse has reached the point of near-impossibility. New York all but sewed up the National League East with a 6-4 win over the Reds on Thursday at Great American Ball Park; combined with the Nationals’ loss earlier in the day, it grew the Mets’ division lead to 7 1/2 games with nine to play while dropping their magic number to 3.