Yes. The game won't work if you use one deck and deal out the cards.

Only 1 Landmark can be in play at a time. You may replace the Landmark at the beginning of your turn before playing any other cards.

 Yes. You can play as many neighborhoods in your city as you want. It is a good strategy to have multiple neighborhoods in play at a time. 

 The Mummer allows you to steal any Public Servant in play from an opponent. When you play your Mummer, move their Public Servant to your side of the table. You will gain that Public Servant's resources and abilities beginning on your next turn, as long as your Mummer is still in the game. You may only steal a Public Servant that is IN PLAY - you never take anything from the hand of an opponent. If your Mummer is sent to your discard pile, give the Public Servant back to its owner. The Public Servant remains in play. Remember, after the game is over, return all cards to their owner.