Canon contradicts you: In book four, the house-elf Winky was able to conjure the dark mark with the use of a wand despite presumably never having wielded one before.
I believe this is a misreading; Winky was there, but the Dark Mark was cast by Barry Crouch Jr. From the climax of Book 4, towards the end of Chapter 35:
I wanted to attack them for their disloyalty to my master. My father had left the
tent; he had gone to free the Muggles. Winky was afraid to see me so angry. She
used her own brand of magic to bind me to her. She pulled me from the tent,
pulled me into the forest, away from the Death Eaters. I tried to hold her back. I
wanted to return to the campsite. I wanted to show those Death Eaters what loyalty
to the Dark Lord meant, and to punish them for their lack of it. I used the stolen
wand to cast the Dark Mark into the sky.
This still shows us that people found it plausible that Winky cast a spell using a wand. (Of course, these were far from disinterested people, plus people are stupider in canon.)
While this is true, Winky is not a representative case for magical creatures in general, since house elves are powerful magic-users in their own right (though admittedly their magic doesn’t appear spell-based).
Canon contradicts you: In book four, the house-elf Winky was able to conjure the dark mark with the use of a wand despite presumably never having wielded one before.
I believe this is a misreading; Winky was there, but the Dark Mark was cast by Barry Crouch Jr. From the climax of Book 4, towards the end of Chapter 35:
You are entirely correct. I mis-remembered the events of book four.
This still shows us that people found it plausible that Winky cast a spell using a wand. (Of course, these were far from disinterested people, plus people are stupider in canon.)
I don’t even want to come up with a list of things the general population thought plausible at various points in canon; it would be too depressing.
Wasn’t it the invisible Barty Crouch who did the conjuration?
While this is true, Winky is not a representative case for magical creatures in general, since house elves are powerful magic-users in their own right (though admittedly their magic doesn’t appear spell-based).