15. Can I let another player take a turn for me when I am away from the game?
You can, with the stipulation that the account babysitter is not your opponent in any current game. It is common courtesy to announce in game chat that another player will take your turn(s) during your absence. Babysitters should only do what is necessary to take the turn(s) and should not interact with the community, start or join new games (except for ongoing tournaments). Furthermore, you should only take another player's turn if they are in danger of missing a turn, not for the purpose of gaining a tactical advantage.
All this applies to standard games as well as tournament games.
Also, account sitting should be a temporary thing. If somebody plays a complete tournament while account sitting, I would think that would be considered "abuse".