Spoiler:
our culprit seems to hide in the shadows, in the form of deception. As of right now it seems that there were 2 murders, which technically there was. The son killed his father, because of the idea he would inherit the company, this idea was planted by the culprit. She had to know his glutinous ways, and his will to get what he wants(he is a spoiled brat). He failed kill him without notice. The House keeper came and saw him dead, and called the Wife of the victim. She gave strange and specific, instructions to the House Keeper. She had to have noticed the dying message by his head, so why didn't she tell. This is because she covered up for him, and because she doesn't seem to be the compassionate type, probably out of fear of her son in law telling everyone she embezzled the companies money, she covered for him hoping in return he covers her. I'm thinking someone implanted into her mind he knew the truth and for him to keep quiet(in other word he didn't really know). She used the crazy diamond trick to fool the police, and had him hug his father to explain the blood. Because the Mistress wouldn't kill her son, someone else did. This person used another optical illusion, they knew the cake tradition, and knew the son was a glutton. So they arranged the last two cakes in such a way the top one looked bigger than the bottom. This is because the cake is in an arch shape, and even if its equal in size, one looks bigger than the other. Thus the son took the poisoned one and died. Then dropped the empty capsule in the mistresses drink to deceive us in thinking she poisoned him, using the doorknob trick. They also switched house shoes with her. They also had to know her habits and where she kept something (say embezzled funds), took it and replaced it with poison, she probably licked her fingers to turn the page and died. She bought tea in pet bottles, before the incident happened as though she expected it. She called the detective, and was probably the presidents real daughter, the housekeeper. She is our culprit, and probably took the embezzled funds and put it in a bank, that receipt will be evidence.
