[Goilveig]

Originally Posted by plzd0ntkeelme

Sigh, you ignore way too many uncertainties.
Whatever the variety, rifles and shotgun are loud enough to alert an entire building.

I think you overestimate how loud they are. I've frequently heard gunshots through only a single wooden wall when I'd go to my family's cabin and people would practice shooting right outside. And unfortunately I've also heard a gunshot from inside the building I was in; at one of my apartments some idiot two apartments down accidentally fired his rifle while trying to clear a jammed cartridge (thankfully nobody was hurt). They're not overwhelmingly loud; certainly not everyone in the apartment building heard the shot.

The biggest reason everyone in a modern house would hear the shot would be echoing down hallways. The standard cheap interior doors used in most construction are very flimsy, and don't block sound well at all. A mansion would have more solid doors, probably hardwood, and that blocks sound much more effectively. Plus, long corridors and multiple floors make it harder for sound to echo through the halls.

You're assuming way too many things regarding the tea.
How can you be sure that everybody at the heated discussion would remember to drink the tea while still warm?
How can you be sure that when the fatal effect happen to one, all the others will be drowsing.
You're also assuming that everyone drinks tea with the same amount in a period of time. They would all drink different amount in a period of time. Someone who drinks tea more than the others in a period of time, would experience the effect much quicker.

You couldn't be certain (that's why the gun is a backup), but remember the killer would have known the dining habits of the guests too.

Again, you're assuming that all these people won't see nor hear anybody coming in a brightly-lit, spacious, open room.

The killer was probably in the room with them -- someone they know and trust. And the killer would just need to stand behind them and shoot them in the back of the head.

The killer can hit them at vital parts with one shot while they're moving around? This means someone must've been some expert shooter. Again this is a form of assumption to expect a murderer is there with such skills.
Way too many assumptions in your post.

It wouldn't take any particular skill to shoot someone in the head at less that five feet. Pretty much anyone could manage that. It would only become complex if you had to shoot multiple targets in succession.