I hope you will allow me to challenge this witch.
Here is my first humble blue truth:
The culprit is most likely Zen Magpie. But it could have been any of the guests present, with the exception of the detective.
During dinner, they stole one of Dr.Mummies syringes and laced it with deadly poison. They proceeded to hide it in Pointy Error's room.
Later, after the soon-to-be victim returned to his room, the culprit used the strong magnet to move the syringe and make it pierce the victim's neck. It is not specified how strong this magnet was exactly, so it is possible it can reach something inside the room from outside. Zen Magpie's room was next to the victim's, so he could have attracted something through the wall.
The same method could be used to tamper with Kelsie's stopwatch, if it is magnetic. Or maybe this happened by accident. That way, the culprit would have had enough time to clean out the kitchen.
When the room was broken into, the culprit used the commotion to grab and hide the syringe.