No. The DH cannot play a defensive position unless he is brought in for the person for whom he is DH-ing. This is a full substitution and negates the DH status. Should the coach re-enter the original player, he must bat in place of the original DH, and the original DH may not re-enter the game.
actually, they could, all you really need for a game, is a pitcher and a batter, the fielders are just for real games. although the only way for the batter to get out is to get struck out or if the batter hits a "line drive" to the pitcher and he catches it.
It depends on the level of play. Some allow that if the starting pitcher is removed from the mound, but put in a defensive position, he can then come back to pitch later in the game, but only once an inning.
That is the baseball or softball fielding position between second and third base.
A Nickleback is a defensive position in football who is not in on all plays. It is a cornerback position who is considered the fifth defensive back on the line. This position only enters the game when a nickel defense is required.
On defense, a courtesy runner can sub for any position they want the next half inning thy play (if the coach wants) or just come out of the game. They can't, however, sub in again as a courtesy runner, and if taken out of the game from pinch running (not DH or playing a defensive position) they can't come back into the game.
"Starting" player for any game is the player who begins the game playing that position. He may be swapped out later and replaced.
Arguably the pitcher is the most important defensive player because if, in a perfect world, they do not let anyone even hit the ball, they are doing all of the defensive work. The other players, besides the catcher, wouldn't even have to be there. Of course this is just theoretical and even in a perfect game, the pitcher heavily relies on the entire team behind (or in front, in terms of the catcher) him/her.