The power of divinations, charms and such could make trials seem obsolete. Thus requiring all sorts of laws to avoid those things. I think an alternative tact to take would be to simply incorporate all those things into the trials. Consider the f**ked system of justice we have in the USA. THe rich get off because they have great lawyers, innocents are put to death, etc..
Even with mystical powers the truth is by no means decided. Watch Kurosawa's Rashomon if you don't believe me.
Maybe you could have two sides both using whatever tactics were available to them. This could work really well if you had a jury system instead of just judges. Consider the following exchange.
Prosecuter: So members of the jury, you heard the defendent refuse to answer the charge under the zone of truth. You heard the spirits proclaim that the defendent was guilty of great impiety, you've heard the results of our medium's probe of the defendent's mind. Surely you must find him guilty.
Defender: My opponent would like you to believe that all signs point to my client's guilt, but consider this. The court magician himself claims to know spells of suggestion that coudl silence my client rather than let him speak only the truth, can you tell the difference? These spirits say that my client is guilt of impiety, but it is impious to dance with a woman on the holy day and who among us has not done that? The medium claims she read the guilt of the defendent in his mind, but how do we know she is telling the truth. Unless you can read her mind you cannot know. Let us not forget when the priest cast Speak with Dead, the deceased claimed that those her killed her should be punished, that sounds plural to me, why hasn't the prosecution produced a partner for my client? Why? I will tell you, because they know they have the wrong man. You have also heard from a skilled diviner that my client's aura bears the color of green, chaotic no doubt, but chaotic neutral, not evil. Surely if my client was a murderer as the prosecution maintains, his aura would be the black of chaotic evil.
I think that could be cool. It would be quite a challenge for the party if they wanted to 'play it straight" and not fabricate evidence, charm witnesses, and the like, but their opponents were willing to do so. I think that could be pretty cool