Zoe-
You keep saying that it's unprofessional to use the terms you don't like (not those which are incorrect, because that's only your interpretation),
No, it's the interpretation of everyone who actually talks about the terms.
but you haven't responded to my pointing out that Britannica, perhaps the most professional of encyclopedias, calls it Pennsylvania Dutch.
And they call Ockham's Razor Ockham's Razor. My guess is that Britannica uses whatever spelling the authors of the article in question prefer. In any case, as I said, it's reasonable to refer to the people as "Pennsylvania Dutch" if they call themselves that.
Regards,
Erik