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.
The illusion revolves centers on the fact that the culprit actually had access to every door in the west wing: the latches on the deluxe rooms were, in fact, magnets, while the latches on the normal rooms were steel. If necessary, the latches were plated to appear "steel" or "silver".
The culprit also had access to two tools, which would later become one: a magnet and an identically shaped steel bar. With these, the steel bar could be used to manipulate the magnet latches, while the magnet could manipulate the steel latches, while the opposite does not hold true.
So, depending on who the unlucky target ended up being, the culprit could manipulate their door and then dispose of the appropriate tool along with the syringe, leaving behind the tool that could not be used.
Clearing away the remains of dinner is no longer a problem: with access to all the rooms, the culprit simply manipulated Kelsie's stopwatch, making the stretch of time appear shorter than it was.
1.Kelsie = Selkie
2.Toja = Jota
3. Lazer = Zarel
4. Zen Magpie = Zep Enigma
5. Pale Darby = Bad Player
6. Dr. Mummies = Midsummer
7. Mr. Pointy Error = EntropyMirror
8. Chitinous Lithe Wolf = Illusion of the Witch
9. For Nine = Inferno (aka Kojiroh)
10. Ignis = Singi (Singidaiva aka Sigil.c)
11. Uncle Bark = Blackrune
12. Kebur Clan = Blackrune #2
13. Evilest Rum = Multiverse
14. Ax Beakbang = Xak Beanbag
15. Go Aces = Segaco
16. Mad W = DWaM
17. Defamed Seaborg = Fedes Gameboard
18. Greg Hal = ghagler
19. Crank Lube = Blackrune #3
20. Hey Plant = Enthalpy
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.
The rum had been tampered with, introducing a lethal poison into every person's body. This could have been accomplished without breaking the seal, via the use of a syringe.
The goal was to have everyone except one person consume an antidote to the poison, which was introduced via the cupcakes.
This makes the locked room murder trivial: the victim was poisoned by the rum, locked their door, and died in their sleep.
Of course, something went wrong. A couple of people didn't consume the antidote, which would have blow the whole thing wide open. These two people later became the victims of an "attack" by Dr. Mummies: the true purpose of which was introducing the antidote into their bodies. This explains why Ignis, who only consumed a portion of the antidote, was feeling particularly ill later on in the story.
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.