To answer your first question: It goes in order, you can't have two ctive targetmen on the pitch. So if the first one is unavailable the second one becomes the target man and so on.
The second question: Well you see closing down cover everything in this game, a rarely closing down makes them stay in formation longer, thus closing down slower, if you mean you want to close down often but using a slow pace(?) when doing this I must ask why? closing the distance slowly gives the player more time on the ball thus more chance of finding a weakness in your defence while the player comes against him. If you want fast and rarely you've already got it imho. A player on rarely closing down will stay in formation longer but once he does close down it's not some halfhearted walking towards the player closing down...
hope that helps