I'm having the same issue. Here's what I've done:
I've talked with the republic people, have gotten the video from the republic computer, I've talked with the two witnesses and the hotel owner (though I could never break Firlith Me's Persuade of who her sith master was), and the mysterious guy. I've killed everyone/thing in the Sith Embassy and I've talked with the wife and the accused. But I can't get anywhere either. I am still getting the whole "Find the real evidence"! And when I flat out said I've got proof, I got more xp than when I said, he confessed and they went along with the trial anyway. Any way I do it, I get people saying that it wasn't true justice.
Any help at all?!?