Your first question is a red herring, as you are trying to make out an investigation is a trial, it isn't.
When the investigation is complete, and charges are laid, you then go to court with your lawyer and witnesses. (The police don't take the POI along with them while they are running an investigation.)
As for your second question, I would comply with any court order, if I believed the order was in error, I would take it up with the court that issued the order. What I wouldn't do is ignore the order, nor would I instruct others to ignore the order.
In relation to Trump, he had the option to allow witness to give testimony in the investigation, he chose to order them not to cooperate. Privilege does not mean subpoena's can be ignored, Trump can use privilege as a means to restrict what they say, but it does not mean the witnesses are exempt from appearing as per the subpoenas.
