Designing a parlor whodunit for a castle holiday in Ireland

Game designers of tildes, lend me your aid.

My BIL, the life-insurance salesman, has decided to splurge on this year's family reunion and he is renting a castle in SW Ireland next summer. I have been given the task of designing and running a murder-mystery game on one night for the ~20 people who will be staying there. Salient facts:

I refer to my inlaws as the barking and snapping (Irish surname)'s. Hilarious but sarcastic to a fault. They have little patience for depth or lore or much historical detail. This will most successfully be a game of nasty betrayals and sudden twists and turns.

There's something like 18 rooms in the castle, with extensive grounds. But alas we will be there at the beginning of July so there will most likely be a lack of atmospheric ground fog. I love a good outdoor setting, so I will most likely try to get people running through the woods and across the lawns.

The family loves their games and takes them seriously but I figure this shouldn't last more than a couple hours or they will lose focus.

I haven't conducted one of these before but I have plenty of relevant experience. I want to keep it simple with a streamlined and elegant ruleset... but it's important to also find a moment somewhere near the end to scare the living shit out of all of them.

What I have so far: Discovering the Dead Body kicks it off. A couple obvious clues and a couple less-obvious ones will get people haring off into different rooms.

Each "player" has two modes. If the lighting is normal then they are playing as themselves and actively searching for the killer. If the lighting, however, is blacklight and the person within wears a black veil then they are ghosts, and must share a clue from a list on a card. All players will be ghosts at some point, including the Dead Body. If they enter a room with blacklight, on goes the veil.

I want to incorporate the history of the locale, as well as that of deeper legends. I mean, it's fucking Ireland. But I don't want the Disney version. One of my most successful history videos is about ancient Ireland so I'd love to incorporate something of the actual historical record instead of just a bunch of Celtic fantasy bullshit.

Some will be drunk, some sober. All will be barking and snapping. Piling on some poor defenseless unfortunate (usually one of the younger aunts or uncles or cousins) is their favorite entertainment. Subplots that subvert that bullying dynamic are also welcome.

Have any of you created an experience like this? How did you approach it? What kinds of ideas does it inspire in you?