OK, This play-through I am doing a ghost run and not killing anyone.
I am now at the Boyd Party and I read somewhere you could do it in a non-lethal way.

I read that you need to talk to Lord Brisby, The guy with the creepy rabbit mask.
The problem is that I can't talk to him. Do I have to talk to someone else first?

Huh, that's weird, I guess.
Normally, when approaching him, he will tell you that he needs to speak to you privately.
He will then move a couple meters and a conversation will start.

You may just have a bug :/

Are you sure you got the right guy? He's in a back room.

Just keep talking to male guests... should find the right one sooner or later.

You may need to fulfill subgoal "talk to all guests" first. Or you may need to find out wich one is the target before next step. Just a guess.

There is a woman in the study to the left as you enter. Asks for a drink and gives you the info on the sisters. Once you speak to her then go talk to the guy. Get entry upstairs and into her bedroom, she likes musicians, small talk her about music in private then follow to cellar. When alone knock out and carry to man waiting on boat.
Non-lethal complete.

That's only for Lydia.
Fact is, it's not always Lydia. So she doesn't always have a thing for musicans.
Also, when I got Lydia on one run, she didn't go the cellar to speak "privately", but she went to the most logical place if she was expecting you to be a musician : in the music room, where there is a piano.

Never had trouble, and I've done the mission twice. Second time I managed totally nonlethal (rare for me; I usually have a few guys pull swords on me and have to feed them to my ratties) and undetected.