Who plays the part of an adjudicator and administrator in a court case?
the plays the part of an adjudicator as well as an administrator in a court case.

The judge is the adjudicator in a court. The judge’s responsibility in an adversarial trial system is to make sure that that the proceedings are legal, impartial and that the rights of the defendant and that the prosecution and the defense parties respect and follow their duties in accordance to the law. The judge also supervises the jury and makes sure that the jury complies with the duties of due diligence and impartiality.  

what is the tax base for the sales tax?

it should be (usually ) 6%