. . . and is appropriate only whenever circumstances arise that cast substantial doubt upon the fairness or impartiality of the trial.
Because the underlying premise is wrong (she was a co-judge) I'm sure that a law clerk's personal business doesn't effect the fairness of the trial.
The only thing that might have created the circumstances that cast "substantial doubt" are all Team Trump's attacks on the clerk, and in my world (not NY) that can't be a basis for a mistrial.