kyon did it because remember the bad guys can control kyon just like when the mansion was attacked and kyon stabbed lucaon in the eye and the bad guy probably controlled kyon to put something in the tea while kyon thinking it was a ingredeant . this is my theory
then who put vs solution to ingredient since you said kyon didn't realize it? and as i remember kyon accidentally hurt lucaon bcs there was a zero behind him and when lucaon grabbed his hand kyon thought it was a zero not knowing that the zero had been killed by lucaon, that's why he instinctively struck back and wound up hurting lucaon
but what if the bad guys can control them fully and erase their memories since they all secretly have that tattoo thingy hiding in kyon's neck for all we know they can just press a button and they could not remember anything. then again another theory
i've got that revolting feeling since christoph suggested Kyon to have a tea break with Lucaon. His expression just gave me a bad hunch. well, i hope it'll be happy endingლ(´ڡ`ლ)