Petry is the second best defenseman on the team by a fairly large margin, but he should play on the second pairing because Subban is the second best defenseman in the world. I would also rather have them on separate pairings so one of Petry or Subban is on the ice for 45-50 minutes a game.
Not the way Petry played this year, Markov is still #2 until proven otherwise.