So, basically what do we have:
- Imperius curse
- Terror tactics
- Legillemency / Occlumency
- Veritaserum
- Cruciatus curse
- Memory extraction spells + Pensieves
- Obliviation
- Confundus
- Priori Incantatem
How the hell do you prove/disprove anyone's guilt? Or an even better question - why didn't Wizengamot's triggerhappies actually use all (or a combination) of the above-mentioned tools to extract the truth from the people they judged? I'm not suggesting that everything should start with a good-ol Crucio - we can always begin with something less powerful and dangerous - a Confundus could literally make a person confused and spill the beans. Memory extraction isn't considered harmful too.
The following cases are illustrative for my point:
- How come did Sirius Black end up in jail, when it was soooooo damn easy to check his story;
- How did Lucius Malfoy sell his story about being under Imperius;
- How did Pettigrew and other traitors conceal their allegiance;
- How did Death Eaters conceal their identities from Aurors and Order of Phoenix (who were actual bloody vigilantes and didn't care about laws and stuff as much), when Aurors had a right to use the unforgivable curses. I can easily imagine Mad-Eye doing something along the following: "Imperio. Now tell me everything. Thank you, Avada Kedavra".