Can the ball be Passed or 'Flip Passed' to an empty space?

NO. A Pass must be made to a team mate.

If your scrum half gathers the ball and is immediately tackled, can he still make his 'free' pass?

NO. He is considered to be wrapped up in the tackle and so unable to get the pass away.

When can I pass the ball?

In your turn you can pass at any time. This can be done before, after or during a players movement. A player can move a few spaces, pass and then complete his move. It is also possible for a player to move have the ball passed to him and then continue his movement.