Butler. It's always the butler.
But joke aside, it's a monastery. Most of those guys, if not all of them, are rapists, murderers and thieves. So take your pick, you choice doesn't really matter, as there is none of those given to blame. It only changes if you will fight more enemies.
Yet, to satiate your curiosity, the "real murderer" is Heather. If you pop her cherry she will write a letter. If you read it and read the "payment" letter MC will make a conection and you can even confront the girl about it.
That's weird. You should exit Arenfield by north (like you wish to go to Witch/Mercenary Camp) and a cutscene should play IF Arianna is on her forest day.
Currently it is not necessary for Arianna to go to Rumah and it isn't related to any quest. It only unlocks few sex scenes and great amount out of Arianna's character dialogue lines. After that you can switch between Arianna going to forest or going to Rumah simply by talking to her.