The official reason why Marie Antoinette was beheaded is incest and pedophilia over her son (which is, to say the least, highly unlikely) but it's clear that it was not the point. The point was that she was the former Queen of France and that they needed to kill at least the nuclear family of Louis XVI as a symbol.
But, could she prove herself not guilty and win some time ? Would it have consequences ? And would she be able to die after the Revolution, either in Austria with her family or under the restaured Bourbons in France.
As elements that would help, she used to be very popular, and the people of France were far from being all against her. And his son was highly mistreated before making his testimony against her, that would not be hard to prove.
EDIT : Apparently I made a mistake while trying to remember. She was judged for treason and the pedophilia stuff was just in order to make her look more guilty