In Tax Court (BTW...I do not recommend going there!! it is for "taxpayers" only) you are the Petitioner and auntie is the Respondent.
As far as prosecuting for lack of assessment authority... did they sue the agent individually or auntie?
I have evidence that the agents in my case were not authorized to make any assessments, a FOIA showed they were clerks! You also have to be a GS9 paygrade or above.... the agents in my case were GS7.