Fifth Case Achievement in The Case of the Golden Idol

You technically only need to find the clues required for the solution. Every clue in the game is marked with a ?/! and will turn into a check mark when you fully interact with them to find all the relevant details.

Solution:
Darkhand Steward wanted Edmund Cloudsley dead. Therefore Ada Baker poisoned the tonic and hid the necessary key in a bag. At the dinner party Rose Cubert ingested the poison and died.