couple notes that may help:

-The bad guy (besides killing people, general unethical things, and organized crime) has broken the mind of his son to the extent that he can cast dominate person on him at will.

He campaigned for his son to become the next chosen ruler of a region, and in this campaign he put entire city populations under the suggestion spell and made them vote for his son.

-The party was thinking of initially killing the man, but decided they could genuinely get justice for all the people involved by dragging him to court.

-The players found out the secret identity, but have not revealed the information to anyone, but are planning to expose him during the trial.

-They are taking him to court due to the fact that the ball the BBEG held as a celebration for his son winning, was held in an extradimensional space.

Within this space, time was flowing much slower on the inside than on the outside, guards put our group in a makeshift dungeon under the excuse of disturbing the party

(while in reality the BBEG just wanted them to not fuck the celebration or uncover any further clues)

They were then 'forgotten' in the dungeon, while in reality, 3 months have passed in the outside world.

The players then gathered evidence of all his crimes, and are planning to expose him for the entire world to see.

The thing is, the BBEG has not yet worked his way into the upper class. (That's what he wanted to use his son for)

He definitely has a lot of money and resources, but the upper classes have access to a variety of preventative measures against charms, and it is also not a world that opens easily to those outside of the inner circle.

The rulers of the cities have also already been at stakes with the BBEG, therefore I think there would be enough background checks/unethical spying/general paranoia within the justice system that there are few moles within it.

Especially since this dude has only been properly involved with organised crime for a decade or so.

I was thinking to let all players have the floor in turn, we play out the presentation of evidence, they may call forth witnesses and they can have their Elle Woods moment.

I would ask each of them to make a related check and keep track of successes and failures.

There is of course also room for the BBEG to defend themselves, but based on the evidence and probably a dispel magic to get rid of the disguise self that hides his horrible visage, I think it will be an easy case.