I just finished the Murder Mystery Party so if you have trouble with a part, ask me about it!
Some tips:
Keep notes, write anything down on something if you have a suspicion about the guest
Reread some of the dialogue if you need to read it more carefully to deduce whether them is suspicious or not and if you still have turns in dialogue
Check ALL the rooms just in case you missed something
That's some of the tips I have when doing this minigame