Does Compel give an action token to the already-been-used-once figure, as well as the click of unavoidable damage? The way I read it a character without Willpower would receive two click of damage: one for the push, and a second unavoidable for the Compel.

Since I'm asking Mind Control questions, can a character with the Mental Shield feat choose not to use it at times, and target more than one character?

1) No token is given as it works just like regular MC.

2) Yes, it is not a permanent restriction, do you can cancel it.

Just like with normal Mind Control no token is given to the controlled character: The action they perform while Mind Controlled is a free action.
Yes. Mental Shields, like most feats, is optional: You can choose when to use it and when not to. Keep in mind you have to take feedback damage if you turn it off though.