One of our directors is in breach of fiduciary duties. However, we still have to work with them on other projects. Should we head all our emails to them "without prejudice" ?
 What is a company usually expected to do when a director breaches fiduciary duties?
In a legal context, "without prejudice" should be used in communications where a party is trying to setttle a dispute, but doesn't want the settlement efforts to be used as evidence against it in court. See:
So you shouldn't be marking all communications like this.
As to the second question, I don't know the answer - sorry.