Nobody was the willing subject of an injection.
A fascinating tale for sure, but it seems to be missing something. How would this theory account for the puncture mark found on the victim's neck?
Created after the fact by the culprit during the examination of the body?
An allergic reaction that the culprit purposefully mischaracterized as a puncture mark?
Kelsie would not mistake something else for a puncture mark.
How would the culprit be able to leave a puncture wound on the dead victim without leaving any evidence behind? If anyone had been in possession of a puncturing tool at the time of the body search, or if such a thing had been hidden somewhere in the room, it would have been uncovered during the investigation.
It wouldn't be suspicious if Dr. Mummies had his tools with him at the time. Surely no one would take note of this when it was found during the body search.
If Dr. Mummies had been carrying his tools with him, it would have been noted. However, nobody was found in possession of anything that could possibly leave such a mark on the victim.
Mr. Pointy Error was stung by a bee or some other stinging insect at some point earlier in the day.
Let's just include an insect sting in the definition of "not mistaking a puncture wound for something else". A puncture wound being left specifically by a syringe-like puncturing tool.
Dr. Mummies had stabbed Pointy with a syringe (or just a needle) earlier, unbeknownst to the rest of the group. Maybe he just stealthed it and stabbed him while engaging in some good-natured skinship before dinner, maybe Pointy took a nap (possibly assisted by drugs) during the boat trip to the island and was stabbed then.
If Mr.Pointy Error had displayed such an evidently suspicious mark at dinnertime, it would have been noticed by Kelsie. Of course, nothing on his person was covering his neck- including clothes or make-up.
An attempt at the table impossibility:
This relies on the fact that Kelsie first saw every lock engaged, then watched from the lounge, meaning nobody could have been clearing the table before the ambush, while after the ambush there isn't enough time.
However, Dr. Mummies was hiding in the Kitchen or Dining room, not in his room. He set his own lock using the magnet trick to fool Kelsie. Thus, he could clear away the evidence undisturbed before the ambush. If necessary, he could do half the work before and half after.
A small needle was used and then swallowed?
The witch looks on with a somber expression. The end is nigh. Only a last, feeble line of defense separates the ruthless detectives from exposing the truth.
If the culprit had taken any clearly suspicious movements, such as surreptitiously swallowing something, it would have been noticed by Kelsie and the others.
How would you suggest they managed to pull it off without arousing suspicion?
Instead of swallowing it, he simply stabbed it into his own flesh. The bleeding would be minimal if any, and depending on where he stabbed himself, the body search should have overlooked it.
Or maybe, just maybe...
he put it in his mouth as he facepalmed in response to PD's foolishness after Kelsie had announced that they'd do a body search.