You can serve from any position behind the endline, but must be between the sidelines. A server will get another chance if the ball hits the net AND lands inside the court on the opponents side. If the ball hits the net and travels out of bounds or if the server hits the net consecutively then the server will NOT get another attempt and loses the point. Players must be at least one arm's length from their teammates. If a player attacks the serve, the player's team loses a point.
Players typically specialize in two to three positions on the court.
A team can freely substitute any player to any position with the referee's acknowledgement. Yes, if the ball contacts any part of the body besides the hands and arms, the other team receives a point. This could also be considered a double hit. A team is allowed to contact the ball 3 times - where a block counts as one touch. However, a team is permitted an additional contact if the ball hits the net during their attempt to play the ball over.