Q1.
A defender takes a throw adjacent to his own penalty area and throws it towards his own goalkeeper. The goalkeeper attempts to clear the ball but misses it completely and the ball rolls over the line into the goal. Ho do you restart play?

Answer.
A player can't score directly from a  throw in in either his own or his opponents goal. In this case as the defender put the ball into the net restart with a corner.

Q2.
A player is about to take a long throw into his opponents box when an opposing substitute, who is warming up, deliberately distracts him as he delivers the throw. Do you take any action?

Answer.
Yes you do. Caution the substitute for unsporting behavior and have the throw retaken

Q3.
You signal for a player to come on after injury - but as he puts one foot back in play, he sees an opponent ballooning a clearance towards him. He pulls his foot back, catches the ball and takes a quick throw. Do you intervene?

Answer
No. The player has rejoined the game with your permission – albeit briefly. So he is well within his rights to step back – he does not need your permission to leave the field in these circumstances – catch the ball, and take the throw. Just be sure the ball was fully out of play before he caught it.