I'm a little confused about the location of the roof access. Is it possible to reach it without being seen by cameras? The story does not seem to bring this up as a reason against it, focusing on the keycard instead.
Regardless, I have prepared another theory for the moment:
The culprit is Dr. Arnold. He hid the arrow somewhere in the room, say between two pillows of a couch. The medication he gave Dr. Snell made the victim's back too numb to notice the arrow piercing him when he sat down later. In other words, it was a trap. The arrow did not immediately reach his heart, instead getting ever closer due to movements of the victim.
The mysterious message had been the victim's own doing.
He was the one who kept putting the glass panel on the roof. However, he wouldn't be able to do it that day, as the arrow in his back reached is heart while he was carrying the panel; he dropped and shattered it while dying.
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.