My class is holding a mock trial for Benedict Arnold and attempting to prove him guilty. What questions should I ask him as the prosecutor?

The burden of proof lies on the prosecution to prove him guilty. Your teacher should know that and instruct the class in an accurate understanding of American jurisprudence.

He can say he was coerced - he had no choice. The British already knew everything and made him look guilty. There was someone else working undercover for the British whose made it to look like Benedict Arnold did it.

Or try temporary insanity - from the stress of leadership and fighting. Or that he ate rye infested with the fungus that causes delusions- that was the explanation given as a possibility to explain some 'witching behavior' in the past.